Uhhh, so you basicaly, you need to know how to make an entire skin right ? x) I'm not a pro skin maker, but i can teach you some thing.
First : All the elements you want to skin
(like a slider or a cursor) have a special name, that you have to know. So the easier way to start skinning is to download a famous skin
(like idk whitecat's skin) and look
ALL the elements name, and see what it is. Then you can try to "mix" some skins without creating elemend to start understanding how it's work.
Second : This video learn you the basics, it will not make you a pro but, you can start with this ^^
https://www.youtube.com/watch?v=zHUsWyGSCjoHope i helped you