Right now, we are in a bit of a transition period, with Hour of Devastation on our doorstep but not a ton of new Constructed results to analyze. With that in mind, I want to walk through the steps I take when selecting a deck during this period. This is a very broad topic, so I am going to hit both obvious steps and some that you may be missing or not consciously thinking about.
Take a look at a format like Modern, where there are probably 100 different deck choices you could make and still have the potential to win a big tournament. Formats can be huge with a ton of viable options, so it is important not to let this be too overwhelming. There certainly are players who always play the same deck at every single Modern tournament they enter. I believe this is okay, if you are willing to make adjustments.
I would advise against playing the same 75 at every event, but if you are able to change your list effectively while still playing the same deck consistently, it could be a smart move. Part of narrowing your range is playing to your strengths. There are often certain types of strategies that a player is stronger at piloting. This is a chance to consider limiting yourself to deciding between an aggro, midrange, control, or combo deck.
Play to your strengths. If you are someone that likes a lot of in-game decisions, use that to your advantage. While you might not end up playing the overall "best deck" by narrowing your range like this, you are gaining much more from a gameplay perspective. Of course, there are players who will always simply play what are considered the best decks overall, and while that works for some people it's not always the best choice for everyone.
Another aspect of narrowing a range of decks that is often overlooked by pros but is very real to the everyday player. Many players don't go into an event with the option to play any deck in the format, and this is okay. A great way to prepare to overcome this challenge is to get your testing in early. I have found that if I am able to lock in a deck early on rather than only a few days before the event, I am that much more likely to assemble what I need to play with.
I have been the guy scrambling for cards an hour before an event. There are a few major problems with this approach. It is mentally draining, so it can take away from the amount of effort put into the tournament itself. Even if you are able to find all the cards you need, on-site dealers are likely going to overcharge you, and there is the possibility of not finding a card at all. Cards being sold out on site is a real thing that can happen, and playing a sub-optimal version of the deck you want to play is not going to lead anywhere good.
Playtesting is important, but exactly what is the best way to go about it? The amount of time a player has to prepare for an event is going to vary widely. Some people won't be able to playtest at all, which means you are forced to skip this step. For the players with very little time to test, I don't advise making playtesting a large part of your deck choice decision. Instead, choose a deck you already know is strong, and then get in games to get a better feel for how to pilot it.
For players with more than a couple hours to test, playtesting plays more of a role in deck choice. If you are able to play multiple decks within your range it really will help make the decision much clearer. Personally, when preparing for a large event Magic Online Competitive Leagues are my first resource, but you don't only want to play online. Getting a feel for playing with the cards physical matters, as it helps you remember all the little mechanical things like using dice, announcing life totals, everything that makes playing in paper different from online.
Having a way to keep track of data can be useful. This can give you a better idea of how favorable or unfavorable certain matchups are. When playing many matches it can be easy to lose track of results, without keeping track of them in some way.
Even if you are unable to play lots of games before an event, there is a way of preparing that almost all players take advantage of these days. The internet is a huge resource whether it be looking over the latest decklists, watching a streamer, reading an article, checking price trends on a card or watching a video, there are many ways to take advantage of the information that is out there. There are players who are constantly consuming content, but most players aren't going to have that luxury.
Once you have an idea of what deck or decks you may want to play, keep an eye out for content featuring those decks. There is too much information out there to try and absorb everything. Taking a look at a few different decklists that have done well featuring the deck you plan on playing is also helpful (we curate those lists for you here on TCGplayer). Be aware when going into the tournament that your opponent has access to the same information you do, so be prepared if they play some of the specialized cards against you.
Having friends that play Magic is great, not only socially, but because it can help your game! Asking someone whose opinion you respect, and is also someone you know pretty well, is a great resource (this is different from messaging a pro on Facebook about what deck to play, who you don't know personally). I'm not saying to go pester someone, but if you are asking a friend they shouldn't be bothered, and maybe you can help them out as well.
The best time to consult someone is when you know they have more experience playing a specific deck than you, and you are interested in playing that deck. This way you can get their knowledge and experience with a deck, without playing with it as much. It is possible that you go to consult a friend about a deck, and you receive feedback such as: "I am no longer playing the deck for X reason." Be prepared to hear both positive and negative recommendations.
This is probably the most important part of choosing a deck. There are plenty of players who realize how important sideboarding is, and will allow it to dictate what deck, or even specific list they play. If there is a 75 that has been proven effective and a pro posts a sideboarding guide featuring that exact list, most players are going to play that exact 75, and sideboard the suggested way card for card. Is this a bad approach? Not necessarily, but it is also the easy way out.
I will post sideboarding guides, but they are still only guides. This means that sometimes adaptions need to be made on the fly. An opponent could be sideboarding differently than expected or have some weird card in their main deck that isn't standard, and you have to be able to adapt. Many of my best results have been with transformational sideboard plans. For instance, If I'm playing White-Black Control and I know the opponent is taking out all their removal, I am more likely to want to bring in Kalitas, Traitor of Ghet.
Sideboarding guides that are made public are both a blessing and a curse. They can be helpful in that they give you a good general idea of what to do for the main matchups, but there will always be a curveball that the sideboard guide hasn't prepared you for. There are always more than a handful of decks in format, and it is necessary to be prepared for decks you have never played against and aren't in your sideboard guide.
My advice is to choose a deck you are comfortable with, and bring a sideboarding guide with you to play if it helps. You don't want to go into an event not knowing how to sideboard, that's for sure. Some decks are easier to sideboard with than others, which can be a good thing. If you are playing a combo deck like Ad Nauseum, for instance, your gameplan won't shift that much after sideboarding. There are only a few general decisions, like whether to bring in alternative win conditions or removal for opposing hate cards, that can be applied to many different matchups.
Another route to take is choosing a deck because there is a lot of flexibility in what you have in the sideboard, and that means it is easier to be prepared for the current metagame. For instance, Abzan Midrange is a deck that gives you a lot of flexibility.
The deck that plays lots of general "good cards" main deck, and you can pick and choose hate cards for this sideboard. For instance, it should be fairly obvious which decks to bring in Stony Silence against, and so on. By making your own sideboard rather than copying someone else's, you already know the reasons in your head for picking a specific sideboard card, including why and where you want it.
This is the way you can be one step ahead of everyone else. Having a general idea of what will be the most popular deck at a tournament is important, and then comes figuring out a good way of attacking them. This can mean playing one of the top decks and tuning it a bit for mirror matches, but it can also mean choosing a deck out of left field that happens to be good against the most popular decks. It is definitely riskier choosing a deck that is less popular, but the payoff is there if you get it right.
When choosing a deck that isn't on other player's radars, I recommend putting in a good amount of time playtesting. Other players aren't going to be prepared for you and they won't know how to sideboard, so use the surprise factor to your advantage. Still, while you will likely play the top decks a lot, be prepared to play against rogue decks too; you still need to be able to beat decks that aren't tier one.
This may seem like an irrelevant category, but it isn't! I have found both from playing myself, and watching others, that players will play better with a deck they enjoy. It is easy to lose focus when playing a deck that feels boring.
Thanks for reading,