The VideoLAN Forums

Given a vlt file
What is the best way of learning to use the editor.

Open it?

Then what

How I did it is open a VLT file and look at the items / images / buttons they put in it and take a good look at the properties of all those things.
Then you can open a new instance of the Skin Editor (while keeping the existing skin open in the other instance of the program) and trying it out for yourself bit by bit.

This is a page to keep in mind for when you figured out the basics: http://www.videolan.org/vlc/skinedhlp/boolexpr.html