The Binding of Isaac: Rebirth

To begin the ascent, it is assumed you have earned the following cheevo

which you earn by defeating Mother once. I have a guide on that cheevo as well if you need it.

Start a new run, preferably with a character you want to unlock the tainted version of. I recommend doing it on normal difficulty. Do not go into Corpse to try to defeat Mother, you cannot get to Home and defeat Mother in the same run. Somewhere along the way you will need to leave a trinket in a boss room or treasure room. It is possible to drop a trinket by holding for several seconds. You can do this on any floor including Mom's floor (even Mom's boss room).

Once you're on Mom's floor, you will need a method of teleportation. Anything works, you just need to have something to get you out of Mom's boss room so that you can enter the strange door in the starting room of the floor. If you don't have a method of teleportation, it's no problem. Somewhere on Mom's floor is a guaranteed skull with a tiny X on it. Explode this skull to get a Fool tarot card. Defeat Mom, grab the polaroid or the negative, and then teleport out. If you have earned the boss rush on this run, grab a boss rush item first and then teleport.

Now in the starting room of the floor you can open the strange door. Touch it and it will delete your polaroid or negative (note that you can also use a broken padlock, sharp key, or faded polaroid to open this door, but the polaroid/negative is much more reliable). Now you will begin the ascent.

The ascent takes you back through every floor of your entire run thus far, repopulating each floor with more difficult enemies and removing most of the items you would normally find. Instead of going down, you go up, all the way back Home. This is a new area where Isaac has never been before (at least, during the events of the game). Eventually you will reach his mom's house. You will start in Isaac's room, where you get a locked chest. Sleep in the bed if you need healing, then move south into the first hallway. On the right is a room with a single pickup in it. Keep going into the living room and walk left into the next hallway. Go north into mom's room (don't sleep in the bed yet). Just below her bed is a unique purple chest. On your future runs this will contain a random mom item, but because this is your first time Home, you will get the Red Key! Congrats, there is your cheevo. But before you move on, let's unlock your first tainted character. Go south into the hallway holding the key and on the left wall you will see a faint red outline. Move right next to it and hit your key to open a secret door in the wall. Enter and touch the character inside, that is the tainted character and it is now unlocked for future runs.

Once you sleep in mom's bed, returning to the living room will begin the fight against Dogma and then the Beast, both of which are very challenging. Good luck and enjoy your new character!