Hi Suzi (and all)
I'd hardly call myself an expert yet, but SCAL makes it pretty easy!
I'm going to be using 5 year calendared vinyl from KAPCO local to me in Kent, Ohio. I bought a roll of white to get started, as I plan on doing the same as you are interested in... school groups, clubs, etc. My son has designed a window vinyl for his friend's Dad that is a taxidermist for his business... a really nice deer head with the company name and phone number.
Transfer tape makes the world a happier place... all of those little bits need to stay in place for placement. Squeegie the tape to the vinyl, peel away the backer and squeegie it on to the glass/vehicle/what-have-you. Wide tape is nice, but it isn't cheap. You can reuse it for your own works, but I'm figuring that each design I cut for sale will go out with a matching attached piece of transfer tape.
Multi-color cuts are a pain... figure on a few nasty words as you get the registration lined up as you place the colors together. The vinyl is super-sticky, and it sticks RIGHT NOW! Patience and you can get a nice cut.
If you are doing a two color cutout of lettering or what-have-you, I find I'd rather cut lettering out of the top layer and have a sheet of color behind for the lettering/etc to shine through. A more solid sheet of vinyl on top is less likely to lose a letter than if you stick individual letters (like the cuts above) on top of a background color. What I'm saying is that if I wanted a red Honda wing on a white background I would cut the wing out of the white and put it on a solid sheet of red behind. You could cut a smaller sheet than the entire cut size, but vinyl will show where there are layers, so I think it looks better if you use the whole sheet... it isn't that expensive... if you are really brave cut the same pattern out of both sheets and piece them back together... we did it for a GM logo (the blue square with the white letters and underscore) and it was a pain... we did it a second time with a square of white behind but there is a line showing around where the square ends...
Hopefully I didn't bore anyone into falling onto their keyboards asleep... if you think of any other questions feel free to ask!