Sometimes it is easier to just look at an example than to read a documentation when trying to understand how something works.
With "skin", you probably refer to the files found at planeshift/art/skins. Notice these zip archives may also contain xml files that define the widgets and have their defaults located in/planeshift/data/gui.
For the technical explanation:
http://planeshift.ezpcusa.com/pswiki/index.php?title=PAWS_specification.
Otherwise, just give it a try and ask people more specifically if you run into problems.
