Step 1: Unlock all of the characters. Unlocking all of them will grant you the ability to use psi rockin with the use key. (If you're having trouble you can ask the hint guy.)

Step 2: Go to Agerate's house at night and follow him down through the cave to the Mani Mani statue.

Step 3: Use psi rockin on the Mani Mani statue. Doing so will teleport you to the ending. (I don't want to spoil the ending though so go see it for yourself.)

I hope this guide was at least somewhat helpful!