Through a script, a specific code is written for changing specific bytes in ram memory.
As simple as that :P
No...I'd like to explain better but it would be too complicated to write it as "step-to-step" tutorial. You'll have to learn how to hack and learn it on your own.
Hey Miksy91 could you tell me how Coolboyman made the change character AND the change colour script in Pokemon Prism. On Youtube you said: "While doing this, I figured out how KBM implemented the colour & character changing in Prism :D" Please could you tell me what to do!