Everyone always seems to want sideboard guides posted with every deck, so they can just pick it up and play without needing to test and figure out the bad cards in each matchup or whether they need to change their strategic positioning. I can relate to that. A sideboard guide can be really helpful when I don't already know a deck well, but I consider it a crutch to use when I'm just picking up a deck. I would never want to lean too heavily on a sideboard guide in an actual tournament. I think it's much better and more important to just understand sideboarding in general, which can help for all those frustrating times when an author is too lazy to write a sideboarding guide for you, or when you play against an unexpected deck, or when your opponent does something you didn't expect, or when you're building your own deck/sideboard, or when you're playing Limited, or... I hope you get it. Understanding how to sideboard is a massively important skill that probably doesn't get written about enough.
Most people probably think the most basic level zero sideboarding, and perhaps the essence of what sideboarding is for, is cutting dead/weak/poorly positioned cards for hate cards. That actually is where everyone starts, I suppose. It's simple and easy, and uses the basic idea of what sideboards are for.
I'm interested in something a little more nuanced, but still broad and abstract enough that it can be widely applied to almost all sideboarding. Astute readers may know where I'm going with this, as it's something I've discussed fairly recently. Sideboarding is more than changing a few cards. When you and your opponent change a few cards, the fundamental texture of the games change as well, and understanding a preparing for that is a big part of successful sideboarding. It's easy to sideboard against your opponent's game 1 deck, only to find that what matters has changed and the cards you've brought in might not even be good anymore.
It's important to understand the general principle at work here. In a vast majority of cases, decks will be built to follow a proactive strategy in game 1. They'll have interactive elements, but those cards will be able to interact on as wide an axis as possible. Most decks will be built to tell the same narrative regardless of what they're playing against, and game 1 will be about letting your deck resolve its narrative rather than letting your opponent do the same.
For the second game, both players get to attempt to rewrite their narrative in the context of the narrative of the matchup, and you want to tell a narrative about how your deck will beat their deck, but you need to be sure you're beating their new narrative, not their old narrative.
You'll notice that I'm framing this in a way you might not be familiar with, but I think it's a useful way to be able to think about games of Magic. Each deck tells a story--Zoo might be, "Play a large threat, remove opposing blockers, burn an opponent out to end the game quickly." U/B Control might be, "Trade cards with my opponent to buy time to take over the game with card draw spells to get to a point where I can counter all of their threats, then deploy a resilient victory condition and coast to a win." Sneak and Show would obviously be, "Use library manipulation to build a hand that can put a giant monster into play. Kill my opponent with my giant monster."
When sideboarding, you need to identify whether your baseline narrative can compete with your opponent's narrative. If it can't, you'll obviously need to work harder to change the story you're telling. Sometimes your new story will be very simple and very different from your old story--against Manaless Dredge in Legacy, most decks hope for the narrative to become: Play Grafdigger's Cage or some other hate card to end my opponent's narrative. Win with whatever.
The most important thing to understand when sideboarding for Constructed, and when building your sideboard for Constructed, is that games 2 and 3 will be slower and more interactive than game 1. Every deck has to be built more or less to accomplish its primary objective as efficiently as possible. When you don't know what you're up against, a proactive strategy gives you the best chance of dictating the narrative of the game. Once you know what you're opponent is playing, you'll be able to bring in hyper efficient interactive cards, allowing you to shift your focus toward disruption. Across almost all matchups and formats, sideboard games will end later on average, meaning that card draw, more expensive spells, and better lategame cards will be at their best in games 2 and 3.
This is why I sideboard additional Chandra, Pyromasters into my Standard deck against literally every single opponent. The nature of Chandra, Pyromaster is such that it's better in games 2 and 3--it's a slow, lategame card that draws cards. This works perfectly with every sideboard strategy I have, because it fundamentally plays the game that people play after sideboarding.
While this is the most important, most fundamental shift you can expect to witness in sideboarding, it's far from the whole story.
Let me shift focus a bit and talk about sideboarding in Limited. I may be best known as a deckbuilder at the moment, but the single element of Magic that I think I'm best at is actually sideboarding in Limited. Interestingly, this is something you never really see--I imagine very few people know much about how others sideboard in Limited. It's difficult to write about sideboarding with a general card pool, so there are few articles about it and no sideboard guides. People watch far more Constructed matches on Twitch than Limited matches, and when they do watch Limited matches, they don't usually (if ever) get to see how players sideboard. As a result, it's really hard to learn about sideboarding in Limited without just playing a lot of Limited. I'm going to try to address that to the best of my ability.
In Draft, just like in Constructed, most decks are trying to tell a specific narrative. For less focused decks, it might just be that they're hoping to hit their lands on time, curve out reasonable threats, and maybe answer their opponent's best creature, but some decks tell a much more streamlined story.
When drafting, there's a good chance you won't have the card pool to radically change what your deck is doing. I love formats where there are enough good cards that you can draft entire sideboard strategies, like Vintage Masters and Modern Masters, but for the most part, that's the exception. In Sealed Deck, however, this is absolutely essential.
Imagine I'm playing Khans of Tarkir Sealed, and I have a good Mardu deck with a consistent plan. I'm trying to "go wide" with tokens and win with Rush of Battle, Trumpet Blast, or Raiders' Spoils. If my opponent's plan is to lock up the ground with walls and then play a big threat like Woolly Loxodon, their narrative will be horribly positioned against mine. Walls don't stop tokens well, as they can only block a single one, and they still let me raid. Investing a lot of mana into a huge creature that can only block a single attacker or that I can easily chump block is much less effective than investing a medium amount of mana into a medium sized creature that will still trump my small creatures, but do so much faster.
My opponent might lose, and they might realize that their deck is poorly positioned. They might cut Archers' Parapet for Jeskai Student to give themselves a more appropriate early blocker, and maybe they get a little creative and bring in a fringe hate card like Heart-Piercer Bow. Now they might have access to tools that can beat me, but they haven't changed their fundamental narrative, and mine is still much better, so I'm likely to win anyway. Instead, maybe they realize, "wow, that was a terrible matchup," maybe they had another build of their pool in mind, so they switch to their B deck. If their B deck has the plan, "play a lot of removal spells, draw cards with Bitter Revelation, win with bombs," they'll find that while they've changed the matchup, they haven't actually improved it. Spot removal is still bad against tokens, and the card draw is too slow.
My opponent needs to understand what kinds of narratives are best suited to trumping the narrative my deck is telling. In their spot, I'd look to be aggressive with relatively cheap creatures that have high power and more than one toughness. They want to try to either end the game before I can build up, or, more likely, apply enough pressure that I have to trade creatures; but because my creatures are small when they're not attacking, I'm likely going to have to make unfavorable trades. This is likely a worse deck than the deck they originally built, since they'd be playing this from the beginning, but a weak deck that's doing the right things is likely better positioned in the matchup than a strong deck that's doing the wrong things.
Sometimes, you can't change as much as you'd like. In the World Championships, in Khans of Tarkir Draft, my deck was all removal spells and card draw with a few big threats. My plan looked very much like the plan of the B deck described above. When I played against a U/W aggro deck with a very low curve and multiple Treasure Cruises, I knew my plan was horribly positioned. It was Khans of Tarkir Draft, meaning that, when I wasn't taking spells for my deck, I was taking mana fixing, and I didn't have room to change what my deck did. The narratives were so lopsided that I felt like I needed a way to "make something happen," so I brought Rush of Battle into my deck where that card made no sense, because I could imagine stealing a game with it. My opponent was trying to race with evasive creatures and would be largely ignoring my creatures, so I could easily get a few of them in play, and trying to play a long game was wrong. I needed to try to steal an early game, so I brought in an out of place Rush of Battle and won the match because of it (with a little help from my opponent). These kinds of desperation plays come up rarely, but it's still important to know what to look for and when you have to get creative.
In Vintage Masters at the World Champions, I drafted a U/G Madness deck that had a sweet white splash for Breath of Life to reanimate a few giant creatures. The splash was well supported, and it gave the deck an exciting, unusual angle of attack. In my matches, I played against Storm, B/W evasive aggro, and Mono-Red Goblins. Every round, I sided out my entire reanimation package. It made me feel a little stupid for bothering to draft all this stuff, but in each match, it was clear that it just wasn't what I wanted to be doing. I needed cheap creatures against Storm, especially since my opponent had multiple bounce spells; it just wasn't worth all the set up compared to just playing an attacker earlier to establish a clock. Against B/W, my opponent had reanimate, so if I discarded a big creature, he could take it. He also had ways to exile my threats, or he could just chump block while killing me with evasive threats--it was a similar issue to what I was discussing above, the mismatch of big creatures against tokens. Small evasive creatures function similarly to tokens, and I just didn't need a body that big. Again, against goblins, I didn't need a body that big, because my opponent was trying to go wide.
All three matches forced me to abandon one of the fundamental narratives of my deck, and it was important that I was willing to do that. I didn't get hung up on letting my deck do a cool thing because it could, I tried to reshape my deck to do the things that would work against what my opponents were doing.
Getting back to Constructed, before the top 8 of the Season Four Invitational, Dan Jessup mentioned that his deck was relatively good against me compared to other Abzan Midrange decks because the threat of Jeskai Tokens had forced him to include more Bile Blights and Drown in Sorrows in his sideboard. I just smiled, because I wasn't really convinced Drown in Sorrow was a good card against me after sideboarding. In game 1, it would be great--one of my advantages was that I knew he had no sweepers, and he had few blockers, so my tokens could be a huge problem for him. After sideboarding, I expected him to overload on sweepers, and I needed to make room for answers, so I was taking out all of my token makers. Now, Drown in Sorrow was a clunky removal spell that only answered certain threats, and which would encourage him to try to hold it for a spot that would never come, making it even worse than it would be if he knew to just cash it in to kill a Seeker of the Way if the opportunity presented itself.
Unfortunately for me, Dan had figured out when we played before that his deck was in danger of not being threat dense enough against me because I had so much removal after board, and brought in more planeswalkers instead of Drown in Sorrows. This is a spot where he was rewarded for recognizing the way that I was changing my narrative, and avoiding bringing in a card that's only good in game 1.
This actually happens to be a common problem with Drown in Sorrow. It's a "sideboard card" in that it answers a narrow enough set of things that you often can't afford to put it in a maindeck, but it actually gets a lot worse in games 2 and 3 in most of the matchups where you want it. It's still a high impact enough card that it's usually worth bringing in there, but it's not the powerhouse it would have been in game 1.
The primary points I hope everyone takes away from this:
1. Sideboard games are different from maindeck games in a consistent and predictable way. Be prepared for that, and understand that it's okay to have cards that you board in or out against absolutely everyone because some cards really are only good in game 1s or only good in sideboard games.
2. Think of games as a story of competing narrative lines where you're trying to tell your story over your opponent's story. Figure out if it makes sense that your story would come out on top, and recognize when you need to find a new story and how to do that and how to match stories well.
3. Predict your opponents' new narrative and answer that rather than their original narrative.
Sideboarding isn't easy, and how you want to sideboard against two different Abzan decks might be totally different based on a few card choices, or even based on how your opponent plays, but I hope I've helped you work out some structures that will make coming to conclusions about what you should be doing substantially easier.