For a stencil: Create the design you want, then print it out. Stick the print out to a piece of card. Use a knife to cut out your design. Put it on your board, get you spray paint colour of choice, and spray a few layers over the stencil. Take stencil off, and you have your design. Then leave the skate to dry.
Remember, lots of thin layers are better than one thick one.
there is actually allready made stencil's you can download of the CWB site, you might want to take a look, to get the idea.