I need help

[DestyGod]

I don't know what I should do to open the door after defeating Mom's foot and unlocking the alternate characters. I already unlocked "A Secret Exit" but when I defeat Mom's foot and use a letter to go to the first room, the door did not appear.

[Sirius]

If you're referring to the door in Depths 2, it's in the first room you spawn in after getting to that floor. There's a marked skull on the floor that is guaranteed to give you a 'The Fool' card. After you beat Mom's Foot, pick up the Polaroid/the Negative and then use 'The Fool' to warp back to the starting room and then interact with the door. Your Polaroid/Negative will be used up and the door will open. The game will guide you the rest of the way.
If you are referring to unlocking the Tainted characters, then the best way to do that is to drop a trinket in the same room that Dad's Note is found in. After dropping it, pick up Dad's Note and the trinket on the ground will become a cracked key. You can then carry that cracked key to the final chapter where you have to use it in the hallway before the door to Isaac's parents' bedroom. After you open the door up, you just have to touch the character inside to get the ''alternate'' version of the character you used to get there.