I posed this question a few days ago as a comment and was encouraged to make it into a standalone post. I'm asking it specifically within the context of Commander, but suggesting your workflows for other formats would probably be helpful for people who aren't me :P
As someone who has been casually playing MTG for several years, I'm only now starting to try to build my own EDH decks from scratch (as opposed to just buying and tweaking precons). I've tried to do my due diligence and research important topics like ramping & mana bases, read articles & posts about determining wincons & threats, and have scoured through EDHREC and Skryfall for thematic/synergistic cards... And all of that is great for finding cards that *could * work in a deck.
But this is the part that most articles & instructional pieces stop at (or glaze over). So now I have a giant pile of theoretical cards for a theoretical deck, and no idea which ones I should actually purchase or playtest with. There is no one-size-fits-all method for paring down your deck, so I'm hoping to hear how you, personally, go about doing it (and whether or not you've come across articles that address this part).
Currently I’m trying to use tags on Moxfield but it’s mostly a confusing mess as I try to trim down ~200 possibilities into a lean, functioning deck. Tags seem a bit too inflexible when I'm trying to tag by both function (ramp, threat, protection) and priority.
Big thanks to Mike, Andrew, gildedjake, and LovesTha BGU who have already chipped in some ideas at this comment. I'll leave it up to them whether or not to repost their comments here.
I don't think I've been playing for much longer than you have, but I have brewed some decks from scratch that can hold their own against my more experienced friends. I use Archidekt's categories feature to organize my candidate cards by function, but I choose functions specifically related to my gameplan. So for example I'm working on a [[Ravenous Squirrel]] PDH deck and I started with a little fewer than 200 cards. I decided I needed creatures that want to stay on the field because they grow or generate repeatable materials (around 12), and creatures that want to be sacrificed (around 23). Then I split the remaining slots up roughly evenly into noncreature spells that provide removal, noncreature spells that give my big guys evasion, noncreature spells that return my little guys from the graveyard, and artifacts that want to be sacrificed. Once I have a sense of these categories and roughly what size I want them to be, it's a lot easier to successively cull the weaker cards within them, or pick out the top ones. All of the cuts I put in the maybeboard so after some playtesting I don't have to find them again if I think my proportions were off or a card doesn't work out like I thought it would.
[[card name]] to call