How the fuck do you mod a ds?????

Bought a Nintendo DSI XL from my friends a few years ago, and have been on and off about wanting to mod it, but anything involving fucking with code scares the shit out of me. I've gathered a few YouTube tutorials, a few wikis, and an online friend who's done it once before, other then that I'm diving neck deep into this. I mean all I want is some custom wallpapers/themes and HShop so I can get games, can't be that complicated can it?

... wish me luck ...