I did a newbie theme when I started making my own DS/R4 themes, tried to replicate the mood of it as well.
Ndsthemes was a great site, easy to use as well(did have search and categorised functions)
Using the sizes/area approximations of the Generic R4 skins is a good way to get started making your own, but make sure you change some of the whites if you don't have Photoshop. TBH, we should make our own skin-producing thread/community for myDS.(Especially specific/custom skins)