How to add custom skins on an original DS character

[Gellybean]

Since I'm not planning to make a whole character by scratch and instead just planning to make a custom skin for a character that is in the game, how would this work and what would I need to do? I never really done this before so giving me tutorial links and an explanation would help a lot, thanks! (Idk if this is even gonna be a mod but I'm sure I'll need the tools for it)