The first place I would start would be to refer to the Concepts & Examples Guides. They can be downloaded here:
http://www.juniper.net/techpubs/software/screenos/
In particular, refer to Volume 2, Fundamentals. There is a section devoted to traffic shaping there. The other piece of this is you need to know what ports/protocols Skype uses. A google search should be helpful here. I personally have not used it, but from what I have heard, Skype may use ports 80 and 443 as well as whatever other custom ports. That may make traffic shaping difficult since you may not be able to distinguish between Skype traffic and regular HTTP/HTTPS traffic.
In any case, start with the guides first and if you have any specific questions, feel free to ask.
-Richard