Find a few clone recipes of commercial beers I think are close to what I'm aiming for, kinda average them together, then fudge grain, mash temps, hops, etc. from there. I've got my own spreadsheet for playing with gravity and mash efficiency/volumes, I'll use Brewer's Friend to estimate IBUs an SRM, but rarely end up making decisions there. For me, it's more of an empirical process over several iterations of the same recipe, more of a "well, this needs to be a bit lighter, and could use a substantially larger bittering charge, better make a note for next time I brew it" kinda thing. It's not a very good process for getting excellent beer on the first try, but screwing around with recipes with the goal of gradually evolving something that's both great and 100% mine is a good chunk of the fun.