Hi:
I've just gotten into the beginning stages of making my own decals. I would suggest you look at this link:
http://www.tangopapadecals.com/Decal%20Talk%20and%20Article.doc
This is a great "how to" article (in Word format) for beginners, and the site's owner, Tom Prestia, is highly experienced. He has many supplies available, his service is excellent, and he has even made perfect custom decals for me. His work is 100% professional, and it's not too expensive.
I recommend you check it out.