Its the same thing as recoloring except you use original characters. To get started, take another character and modify it's apperance (not recoloring) so that is has a new gun, arm, head, etc. This will help you get the feel for what looks good and what doesn't. After you do that enough, think up an idea for a character and try to use all the elements you have experimented with. If you ever get stuck, look at someone's work to get a feel for what looks good.