Brewing decks is something that has no real hard and fast set of rules. Almost by definition, each brewer is going to approach things in their own distinct way. This is going to lead to individual styles of the brewing process, of the resulting decks, and even in the play-style of that player. This is all pretty awesome, but as a result it can be quite intimidating to ever venture off into brew-land for the first time yourself. You have no obvious path or reason to brew necessarily. The deck that won last week's Grand Prix has been crushing your FNM just fine. You have a curiosity, but the task seems like such a departure that you stay safe, sticking to what you know.
Brewing is a highly customized world, so your starter kit doesn't need to go very deep; after a short while, you will be developing your own habits and style. Still, it can be nice to know a few things that help you kick off the whole process to begin with. Here are some tips and recommendations for transitioning over into the realm of a brewer.
Choose Your Time Wisely
Any time is a good time to Brew in the sense that keeping your brain sharp and in Magic mode is always good, but not all times are created equal when it comes to conditions that favor brews.
The first step to understanding when you can best capitalize on brewing is to understand what kind of brewer you are. Are you someone who enjoys building more proactive strategies like combo decks and aggro decks? If so, you will probably find most of your success right after major format shake-ups, such as set rotations. This is because the format is most up-in-the-air at this point, meaning people will not tend to have streamlined decks that tend to be weaker to extremely proactive and focused strategies.
Alternatively, if you are a deckbuilder that enjoys more reactive strategies or building, such as midrange or control, you will likely find more success after a format has established a bit. This allows you to know what enemies you might encounter and to prepare for them ahead of time, during the deckbuilding process. This, in turn, ensures you have the proper deck composition and can now react to the threats you anticipated ahead of time. You can lean more or less heavily on this and slide from midrange to control, but in all cases, you are making some number of assumptions about what your opponent is playing.
Neither of these styles is more correct than another and it is possible that you fit both roles or neither. The take away here is that brewing is a time consuming process and we want to make it as efficient as possible. Find out what works for you and stick to it.
Isolate Cards with Unrealized Potential
The above is important to effective brewing but the actual first step to brewing is to know what you are going to brew around. Competitive brews are almost always a bottom-up process where a card, or mechanic, or combo is isolated and then built around. Very rarely will a player go, "I want to brew a combo deck that mills the opponent out," and then begin their search for a mill deck. This process would lead to a ton of wasted time as you are effectively playing Go-Fish with the metagame. Instead, what is more likely to happen is that you spot a card like Glimpse the Unthinkable or Increasing Confusion and you go, "Hey, this is a great start to mill, let's explore what else works well in that strategy." Because of this, it is pretty crucial to have some solid building blocks to work off of.
This means we need to be looking at cards that offer us something worth investing in. Generally, this is going to fall under unique cards and powerful cards. If a card is not the most powerful thing in its class, or the only thing in its class, you have to question why you are using it. If the answer is that you already have four copies of the more powerful thing, then proceed as normal, but if you come back with any other answer, make sure you aren't selling yourself on a bad choice.
Synergy exists and you can certainly discover something worth building around that skirts these "rules," but in general we want to make sure we are spending our time productively; barking up tier 3 trees is only fun for so long.
Occasionally there will be powerful cards, the most powerful cards of their class even, that are not seeing play for other reasons. Maybe expensive cards are seen as unplayable or maybe artifacts are just too weak in the format. For example, I played Griselbrand to an 11th place finish at Pro Tour Gatecrash. I don't think anyone would argue that Griselbrand is powerful and most would agree that if you were going to spend 8+ mana, he is probably the best bang for your buck. That said, spending eight mana on a card in Standard at the time was a bit of a stretch, especially one with a bunch of black mana required as well. When your goal is to cast Griselbrand though, it doesn't take long to arrive at Crypt Ghast and thus a deck was born.
Iterate on Foundational Ideas
One of the most critical mistakes that newer deckbuilders will run into is the unconditional commit to foundational ideas. In other words, they are unwilling to modify those base cards that we just talked about in the previous point. Let's take the Griselbrand example and apply it here.
Say that my goal was to cast Griselbrand but Crypt Ghast didn't exist. Instead, the best ramp I can find in the format gets me to seven mana on turn five. I could Insist that Griselbrand remain at the top of my curve and just forfeit that explosive turn five, instead rolling it over until turn six. Or I could go in search of a win condition that costs seven mana and therefore curves perfectly with what I want to be doing. If that search comes up with poor options, we can always revisit Griselbrand, but if I am unwilling to ever look away from Griselbrand, I might miss a much better fit for my deck.
Modifying those cards on the periphery is pretty easy to do as the brewer has low attachment levels to them. It is important to try to be as honest and impartial with your brew as possible right now just because most of your human urges will work in the opposite way. Showing your brew to friends who are willing to talk things through with you is a good way to get some other eyes and opinions on your deck to keep you a bit more Grounded during the brewing process.
Introduce Powerful Pillars
As an extension of that last point, but also breaching over into its own territory, do not be afraid to play good cards in your decks, even if those decks are being built around what most consider a "bad" card. Many brewers can be stubborn. Some refuse to ever play the best deck in a format, even if they cannot come up with something strong enough to compete on their own. This is a really tough thing to change and a topic for another time, but there are some ways to meet in the middle here.
It's awesome that you want to build an Abzan counters deck. Anafenza, Kin-Tree Spirit is a strong card that is seeing basically no play, as is Avatar of the Resolute and High Sentinels of Arashin. By all means, play those cards and other counter synergies, but do not rob yourself. You should absolutely be running Hangarback Walker in this list as it is probably the best card in the format that comes with +1/+1 counters on it. The card is heavily played for a reason and you don't need to avoid it to have something unique in front of you.
Going even further than that, we should not stick to our synergies at all costs. Siege Rhino is a card in our colors and I find it hard to believe that the worst +1/+1 counter card in our deck is better than a Siege Rhino. We don't need to go through and Erase the face of our deck and turn it into Abzan midrange, but we should not ignore powerful cards when they make sense.
This point especially holds true for removal and sideboard staples. There is a reason you tend to see the same removal and the same sideboard cards pop up over and over again and that is because they answer the appropriate threats in the format. If you are changing this up, have a reason. Maybe your Eldrazi ingest deck can play Processor Assault over Roast and that's solid enough, but cutting Wild Slash or Fiery Impulse for Processor Assault starts to tread on uneasy territory as the card functions in such a different way despite the outcome often looking similar.
Identify Strengths and Weaknesses
By this point, you should hopefully be playtesting. Most ideas you have need to be validated through testing or else you are walking into a tournament with a bunch of theories and that is obviously not giving yourself the best chance of winning. During your testing, it is important to keep track of things that your deck is strong against as well as any weakness you spot.
Sometimes, this will come in the form of a specific card. If I am playing Griselbrand and Act of Treason is seeing some amount of play in the format, I should know this and plan accordingly. It does not mean I have to pack up my deck and go home, but it does mean it should be on my radar and I should not be completely surprised if and when this happens. Instead, I should have some sort of strategy in mind to play around this or to at least understand it is a possibility so as to avoid being blindsided.
Other times, this strengths and weakness come in the form of a class of cards or even an archetype. Your deck can be perfectly viable while having a 0% matchup against control as long as control makes up a very small percentage of the metagame. But learning this might streamline your deck in other matchups as now maybe you can Remove useless sideboard slots that don't actually turn your control matchup around and instead can allocate those resources to other matchups.
Solving these weakness and showcasing these strengths comes during multiple stages of your brewing. You should be modifying your deck to minimize known weaknesses and to capitalize on known strengths, but you should also be playing to those cards/strategies in game. If your deck is trying to ramp out a Griselbrand, you probably shouldn't be building an aggressive shell as those two strategies just do not work well with each other. Similarly, even if you built a sweet control shell with Griselbrand in it, you still need to play like that card is in your deck. If you have the option at staying at eight life by using a removal spell on something you normally wouldn't, you probably should. Being at eight life with our copies of Griselbrand in your deck is exponentially better than being at seven or below.
Ultimately, brewing should not be a set of rules and regulations as it needs to be something you enjoy. You can absolutely set your own limits and restrictions for yourself, but the art form as a concept doesn't really place this pressure on you. Brewing competitively does have the end-goal of wanting to win attached, but doing anything competitively has that clause.
Brewing offers players a chance to inject some of their own personal style into the game. It adds an element to the game that allows it to capture even more people. Magic has a little of something for everyone and it is this tapestry that makes the game so beautiful. So if you have been thinking about dipping a toe into the brewer pool, please, the water is quite warm!