From GameBrew - a wiki dedicated to Video Game Homebrew.
DSi4DS looks and feels just like the DSi firmware, but it will run straight from a flash cart on your current DS or DSLite. You can load homebrew games and applications from it.
Commercial games, although shown, currently can not be loaded using DSi4DS.
It was ranked 3rd in the NEO Summer Coding Compo 2009, APP section.
Download and extract file.
Put DSi4DS.NDS to the root of your card.
For the photo setting on the top screen, make your photo 256x192 and name it photo.jpg, then put in the root of you card.
Stylus - Drag the icons left/right
Tap the scrollbar buttons - Scroll icons
D-Pad - Scroll left/right
A - Launch a homebrew