A CycloDS Evo skins consists of several files:
top.bmp
bottom.bmp
menu-icons.bmp
scrollbar.bmp
cheat-icons.bmp
skin.xml
The bmps can have different names, but their entries in the xml file must have matching names.
All the files go in a folder for each skin in the CycloDS\skins folder on your MicroSD card (if you don't have the folder, create it).
For example: G:\CycloDS\skins\skin1\, G:\CycloDS\skins\skin2\, G:\CycloDS\skins\skin3\, etc.
If you're trying to make a new skin, the best way to learn is to look at someone else's skins.