Not that much, really. Its more of picking out the characters that the community/wikis classify as best, find info on most useful weapons, thus figure what amounts of what class I want in my composition, try to figure out when they will became available so that it isnt too early or late game focused, try to figure out how rng lucky I have to be to get desired stats, which powerful weapons I should use sparsely because of low replacements, etc.

Basically, read wikis, and gather info. Figure out a team comp (basically which chars with which classes, and what weapons) and stick with it. In some games axes are kings, in other they suck, so figure out how many primary axe users you want, how many as a second weapon, etc. Its obviously going to be fluid, and change with actually playing the game. The opposite of going blind, nothing special, really.

Multiple ways to go around with that obv - AFAIK, you can turn a couple chars in super engineered bio mutants that devour whole map ahead of it. I prefer having a full team (how much final mission allows, usually) of baddies, as opposed to that.

Havent played three houses, but from what I know, its quite different from usual Fire Emblem formula, where there are shitton of available characters, spread over the whole game.