Haptork,
I was one of the first ones to become a professional skinner. I have made skinning my only profession since 2001, so here are a few advices...
The first step is to get practice. The most free stuff you make for the community , the most you will learn what the users want. Also, a good point would be giving first to ask something later. I did over 300 skins before asking for something in return, but this is something that you should think of what would be a good number.
A second step, after you get a lot of practice, you must have achieved a Master level here in WinCustomize, so you could start making some Master Skins, what would start to give you some income.
Make a website, showing your work would be a good support, for potential clients to see what you have done.
Finally, try a full time job on a skinning or graphic design company.
Well, the jurney is not that easy as many people may think, but it is not impossible and for that, be ready to work very hard.
Good Luck! 
