Makes me kind of sad that Linda (Enayla) hasn't posted anything on MTS2 for almost two years now... damn, that girl had some serious kickass talent.

There's an old skin of hers that I would make into an alien default but I have no idea how to go about doing that, is there a tutorial somewhere?

Easiest way would be to download an already made alien default and switch out the textures with Enayla's textures in SimPE. Don't worry about stepping on the original creator's toes for this. There is only one way to make this and if you did it from scratch it would be exactly the same as what you will end up from this method.