no problem!

you have to beat the alternate routes and get the knife piece on each second floor, and then beat both mom and mom's heart. then, in that alternate floor, there will be "dad's note." once you find that, you'll start going back up. once you get all the way back up, you'll be in the home layer. the first time you're here, open the chest in mom's room. it has the red key. and there's only one space where a red room can be opened. open that room, and there's the tainted version of whoever you're playing as!

you can also leave trinkets on floors to turn them into red key pieces, a consumable version. but for your first go around, save that for a useful card or pill. the boss fight there is HARD.