1. Get a vector-graphics program, like Adobe Illustrator and design whatever you want as a decal, using only a single (or two, or three) colors. You can use other software, as long as it can export a .ai or .eps file.
2. Find a
vinyl cutting machine. Some high school art departments have them, otherwise local graphics and print shops usually have them. (Or, your team can just purchase one for under $500).
3. Using adhesive-backed vinyl film, cut out the vector decals on the vinyl cutter. (If you don't want transparent decals, you can use opaque vinyl).
4. Stick the vinyl decal onto the Lexan*, or whatever you else want. (The vinyl material is adhesive backed, and can last through anything, even outdoor signs).
5. ???
6. Profit!
* If you have a complicated design, get wide masking/painter's tape, and cover the entire cut out vinyl decal. Peel the backing off of the vinyl, with the vinyl decal stuck to the tape. Then stick the vinyl decal and tape together on the target location. Press down firmly, and use something like a roller to ensure the vinyl is stuck firmly down without any air pockets. Then slowly peel the tape off, leaving the decal on the target destination.