step one is of course to pick a commander. commanders can be any legendary creature that isnt on the banlist (updated by wotc) or planeswalkers that say they can be your commander.
step two is to figure out what that commander PRIMARILY does. my favorite commander, zoraline cosmos caller, has two abilities: a graveyard recursion ability and a lifegain ability. i chose to build around the lifegain ability (when a bat you control attacks you gain a life)
in order to capitalize off of this specifically i obviously needed bats. there are bats that trigger things when you gain life (beautiful amazing). add those. other creatures trigger on instances of gaining life (archangel of thune)
in order to maximize gain life triggers, add other stuff that makes you gain life. authority of the consuls is a good one for me personally in this deck, as well as tocasia's welcome (?) which makes you gain life when your creatures hit the field. boom life triggers for your bats to react to
and then utility stuff that goes in white/black decks. i play with lifegain and black has a lot of life resource usage, so i added bolas's citadel to really capitalize on that. i dont have necropotence bc 3 black is a crazy proposition usually but other cards that let you use your life to do shit are great
commander colors have certain staples. white is removal/control, black is life resources and graveyard recursion, blue is control counterspells, green is mana ramp, and red is burn and also nonsense. so make sure you have the staples. for white/black mine are: swords to plowshares, path to exile, banishing light, grasp of fate, oblivion ring, murder, and deadly rollick (for removal, because your opponents want you dead and youve gotta prevent that)
in any color it is ALWAYS good to have a healthy amount of interaction. get rid of stuff you dont want on the board or prevent it from hitting the table. people who bitch and complain about interaction are beta cucks who hate playing competitive games and who just want to play solitaire. i am presuming you know this already i just like to yap
anyway i dont exactly know ur magic the gathering previous knowledge but i hope this helps and also if i said stuff you already know then i'll commit ritual suicide dont even worry about it /silly
synergy with the commander π
also keep in mind your mana curve (cost of cards in your deck) because it will FUCK you if you dont
also 34-36 lands is the sweet spot, more if landfall, less if your mana curve is really low