AFAIK there is no step by step guide. This means a lot of testing and experimentation by your own. Be prepared!
UI Skinning includes xml coding (optionnal: elements size/positionning) and graphic design (tga mainly).
- from scratch: use the latest official skin pack
and good luck!
- from the work of others here (randomly...hum...Adra's) - still need some hard work then but a lot easier -don't forget to promote the original work/author.
Enjoy till it last!