Your 60-card deck works and executes its plan. Now the question changes: how does it perform against the decks you'll actually face, and what should the other 20 cards be?
Troubleshooting specific matchups
If your deck was built to beat a particular hero, start there. Otherwise, start with the decks that are well represented and performing at recent competitive events. The logic is simple: if your deck loses to too many of the top decks, it isn't competitively viable right now, and you want to find that out early rather than after weeks of tuning.
Play your base 60-card deck into the matchup first, even if you already suspect you'll need different tools. Knowing how close the matchup is by default tells you how much sideboard space it deserves, and sometimes the answer is none, because the matchup was already fine.
While you play, look for what's going wrong or right. A few common patterns and the usual responses:
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Your hands convert more efficiently than theirs. Play the value game. Win turn by turn on efficiency and don't overcomplicate it.
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Their hands convert better than yours, and they're aggressive. Lower the value of their turns. Disruption and defensive tools do more for you here than trying to race.
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Their hands convert better than yours, and they're defensive. Move value into future turns instead of spending it now. Setup turns that bank value for a bigger turn later deny a defensive opponent the chance to block efficiently.
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Their on-hit effects are wrecking you. Add defense reactions or extra armor, or play around them. Sometimes the fix isn't a card at all, just declining to leave a card in arsenal when you know what's coming.
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There's a specific weakness to attack. Look for tech cards that line up against the strongest part of their deck rather than generically good cards.
The through line: identify what's actually beating you before deciding what to add. Most sideboard mistakes come from adding cards that address a problem the matchup doesn't have.
Strategy beats card choices
Those fixes are logical and usually work, but they can backfire when they dilute your plan too much. Defensive cards added against an aggressive deck are worthless if they cost you the offensive turns that were keeping you alive. Sometimes the right answer against a punishing deck is to stop defending altogether and convert as hard as you can.
Cards aren't the whole matchup. How you play is often just as important, and usually the answer is a combination of the two. A useful way to find it is to try extreme approaches and see what moves the needle:
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Convert fully on offense every turn and never put a card in arsenal.
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Alternate between small and large hands to control what you can defend with.
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Block every on-hit effect, no matter the cost.
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Play as defensively as possible and try to fatigue them out.
None of these is likely to be the optimal line, but they tell you which direction the matchup wants, and the best approach is usually somewhere in between. What you learn here also feeds back into which sideboard cards you actually want.
Building an ideal 60-card deck for a matchup
For the heroes you expect to face most, or the ones giving you the most trouble, try building a deck dedicated entirely to beating that one deck. Ignore your core strategy and build the best possible list for that single matchup.
This does two things. It tells you whether the matchup is winnable at all. If you can't beat a deck even when your whole 60 is devoted to it, no sideboard is going to fix it, and you should either accept the loss and focus elsewhere or reconsider your hero. It also shows you which cards actually matter against them, and roughly how many sideboard slots the matchup would need.
Once you've built an ideal 60 against each of the top decks, compare them. Cards that appear in every configuration are your core. From there, count how many cards you'd need in total to cover all of them, which will come out well over the 80 the format allows. That's fine as a working state. Many players test with 85 to 90 cards until close to an event, then narrow down. The advantage is that you can try a new card without first solving the puzzle of what to cut. Work out whether a card is worth space first, because finding the space is the easier problem and it can wait.
Start with equipment
When you build an ideal 60-card deck for a matchup, that includes equipment, and equipment is usually where you should start.
Equipment starts in play, so it affects every single game you use it in. A deck card only helps if you draw it, and in a fast game you often won't. Swapping in one defensive card against an aggressive opponent might do nothing at all: you may never see it, and even when you do it gives you slightly more defense than the card it replaced. Swapping one piece of equipment for another changes your defensive floor from the first turn, every game, for one sideboard slot.
That's a much better return. Equipment is also easier to sideboard well, since you're comparing two pieces in a fixed position rather than working out what to cut from your 60.
Blue tech cards
There's another way to answer a matchup without spending sideboard space: find your answer in a blue card and run it in the main deck.
The cost is low. Blues are primarily there to pay for your turns, so swapping one blue for a slightly worse blue that also does something useful barely changes your deck's quality. This is especially true when the card still blocks for 3, since extra blues usually end up defending anyway. You're getting a matchup tool for close to nothing.
That has a consequence for how you build the sideboard. If a blue tech card can live in the main deck as part of your resource base, it shouldn't take up a dedicated slot. Reserve your sideboard for the less flexible, higher-impact cards: reds and equipment. Most decks should look primarily at red sideboard options for that reason, with high-blue decks as the exception.
One caveat: some blue tech cards have 0 defense. Those can still go main deck to save space, but the cost is real, since a card that can't block is a dead draw on defensive turns. When you have the room, those belong in the sideboard.
Building an 80-card deck
Once you know your ideal configuration for each key matchup, the work becomes trimming. An ideal 60 usually overcommits. You might have made a matchup favorable with twelve defensive cards and find on further testing that six get you nearly the same result, which frees six slots for everywhere else.
Trim systematically. Change a small number of cards at a time, usually one playset, and see how much the matchup actually swings. When two cards compete for the same slot, a useful habit is to play one and ask yourself how you'd feel if you'd drawn the other instead. That comparison tells you more than evaluating either card alone.
What you're really measuring is each card's effect on your overall win rate, which comes down to three factors working together:
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How many matchups it's good in. A card that comes in against several heroes is worth more than one that only comes in against a single deck.
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How much better it is than what it replaces. A card that only shifts a matchup slightly isn't worth a slot, however narrowly good it looks.
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How often you expect to face that deck. Recent tournament results give you a picture of the wider field, but for a local event it's worth knowing what people in your area are actually playing.
The biggest efficiency gain comes from cards that work in more than one matchup. Sometimes the right call is to skip the perfect answer from your ideal 60 in favour of a card that's merely good enough here but also does a job somewhere else. Two "good enough" cards covering four matchups usually beat two perfect cards covering two. Overlap is what turns a sideboard package that looks marginal card by card into a genuinely strong use of space.
Locking in the final 80-card deck
Your card count shrinks as testing goes on, but there's no rush. Many players stay at 83 to 85 cards until a week or two before the event, then make the final cuts.
When it's time, lay out the 60 you'd actually play for each matchup, and set the cards you're not playing next to them. That second pile is where the useful information is. If it contains cards you'd happily play in that matchup, you may be over-teched for it, and cutting one of those in favour of something that helps elsewhere costs you almost nothing. It's one of the easier ways to find the last few cuts.
Do this for every deck in the format, not just the ones you built ideal 60s against. A tournament field contains more than the top three decks, and you need a reasonable 60 against all of it.
The final cuts are the hardest, because they come with real trade-offs. You'll often find you can tech for one matchup or another but not both. When that happens, weigh how impactful the remaining cards actually are and which matchup you'd rather have the percentage in. The instinct is to shore up your worst matchup, but turning a solid matchup into a favoured one can be worth more overall. Your goal is win rate against the expected field, which isn't the same thing as fixing your weakest spots. If you haven't found tech that meaningfully improves a matchup, it's usually right to let those cards go and invest the slots elsewhere.
Once the 80 is set, double-check that your key matchups still feel right. If your testing was thorough, this rarely takes more than a game or two.
Finally, redo this process for each tournament, even ones close together. The metagame shifts, and your own evaluations change based on what you learned at the last event. The good news is that it goes much faster the second time, since you know the deck inside out and the changes are usually small.
Core versus sideboard
You'll notice this series talks constantly about 60-card and 80-card decks, and barely at all about finding your deck's "core". That's deliberate.
A useful way to think about it: you don't have a main deck and a sideboard. You have 80 cards that let you build a different 60 for every matchup. The core is whatever ends up in all of those 60s, or all but one. The sideboard is whatever moves in and out. Both labels describe a finished deck rather than steps in building one, and they emerge on their own once you've done the work.
The labels are genuinely useful for explaining a deck to someone else, which is why decklists and guides use them. They just shouldn't drive your decisions while you're building. Once a player decides a card is part of the core, they often stop considering cutting it, even when cutting it would improve a matchup. The better question is never "is this a core card?" but "should this card be in my 60 against this deck?" Ask that one and the core takes care of itself.
Refining the process
The same way a deck is never finished, neither is your method for building one. The concepts stay stable over time, but the process around them keeps getting refined the more you use it.
That's the real value in reading someone else's approach. Take the parts that fit how you think, discard the parts that don't, and let your own process develop from there. Deckbuilding is one of the most rewarding ways to engage with Flesh and Blood, and this guide exists to give you enough of a framework to start enjoying it yourself.
Source: https://fabtcg.com/articles/masterclass-deckbuilding/
