Decide WHY you want to sell your soul (ex; love, money, long life, happiness).
When the sun has gone down, go to the back of your closet, and close the doors behind you.
Repeat these words... "I, [your full name], want to sell my soul in order to [your wish]"
*Side note* If you want to get on Satan's good side, also add in how much you appreciate his work and how much bad he does for everyone.
Sleep in the closet for the night. No matter what you hear outside DO NOT leave the closet until the sun comes up.
Do not forget to leave a peanut butter and jelly sandwich inside your closet every day that you are alive. Do not forget or else everything will break.
You have successfully sold your soul, hope you don't regret it, because there's no turning back.