Purify Chamber

1. Put a Shadow Pokémon in the middle of any of the slots in the chamber 

2. Place up to four other (non Shadow!) Pokémon in the spots around it.
There is now a line going from the Shadow Pokémon to one of the outside Pokémons, and a line tracing a ring through the four outside. each line is either blue or white. 

3. When the line is tracing FROM one Pokémon and going TO a Pokémon who has any single type that is weak to any of the types of the first Pokémon, the blue line will turn white. This means the alignment is good. 

4. Your goal is to set up your types in such a way that all the lines (the whole ring and the line leading out from the Shadow Pokémon) are white.
Doing so maxes out your flow and tempo, which quadruples the purification rate. 

5. The heart gauge of the Shadow Pokémon will now decrease as you run around and do other stuff. 

6. Once a Shadow Pokémon's heart gauge is fully empty, the chamber will tell you you can purify it.
Select the Shadow Pokémon in the chamber and hit the 'purify' option.
This only works from the Purify Chamber itself on Lone Island, not from a normal PC!

7. Once you purify a Shadow Pokémon, it will gain all its EXP it got as a Shadow Pokémon, will forgot all Shadow Moves and will learn pre defined other moves instead.