Basically, it is very simple : use ResHacker to edit the resources of the dll, and change whatever you want. You can add some extra icons too, for specifics folders, they will be displayed when the user requests a "Custom icon" in the context menu.
You can a preview to your skin, just set a png (max size 128*128) in the skin folder. Some informations about the skin (Name, Author, etc) can be set by editing the "Version Info" Section.
NOTE ABOUT NEW FORMAT:_________________________
Since iCF 0.9, you can add a generic folder icon to your skin, that will be used to replace Windows generic folder icon. Skins that have this custom icon have "2.1" as version number (others have "1.1"). All skins provided in the package have this generic icon, however old skins will work as well (but without the extra generic icon).
CONVERT A SKIN FROM OLD FORMAT TO NEW:_________
Actually it is very easy, because there isn't a lot of difference between the two formats. The new is just cleaner and more powerfull. Here are the specs of the new format :
* 1st icon : Red folder
* 2nd icon : Orange Folder
* 3rd icon : Yellow folder
* 4th icon : Green folder
* 5th icon : Blue folder
* 6th icon : Pink folder
* 7th icon : Grey folder
* 8th icon : generic folder
After that, you can add any icon you want, they'll be displayed when the user will choose the "Custom Icon" in the context menu. An unofficial convention says that the icons 9, 10 and 11 should be "My Music", "My Pictures" and "My videos", but it is not an obligation.
