Sorry I don't have any answers to make it easy to do skinning. It takes knowledge of html, css, and getting to know the rendered html from mojoportal to discover the class names and things you can use as selectors for assigning styles. A lot can be learned from the included skins, thats really the best I can do is give good examples, there is no silver bullet I know of to make it all easy.
The css files are fairly self documenting, they are organized so that layout is in one file, colors in another, borders in another, images in another. menu/treeview styling is kept in a single file for the menu or treeview.
Even for me, creating a skin from a design is a trial and error process. I always use an existing skin as a starting point and I choose which skin to use as a starting point by the layout not the colors. For exmple if the design I'm trying to use is a fixed width layout with hirizontal menu, I pick an existing one that is fixed width horizontal menu. If its a fluid layout I start with a fluid layout, etc. Then its a process of making small changes, refresh the browser to see if it worked, repeat until the design is achieved.
Gradually you develop strategy from experience and have a good idea what changes to try, but its still the same process of make small change, see if it worked, repeat.
Hope it helps,