You can get a can of car spray-paint at a local auto store(O'reilly's, Autozone, etc). For the template I would take some measurements of how much space you have on the side of the case. Make a block on a poster-board(not the thick foam ones) that meets the requirements of your case's side. Inside your drawing zone using a pencil make a 2 second layout of your words to get a good idea of size and how to fit it in the area. Then go into detail with the pencil and make it as COOL as you'd like. Once you are satisfied with a certain set of lines you made, take a permanent marker and trace the lines of the letters that you want to use. The next step would be to cut out the letters of your template. I would suggest using a box-cutter knife or an exact-o-knife(probably incorrect spelling). Now that you have your template! You can proceed to the next step which is a lot less forgiving than having to buy more poster-board. Take your template and tape it to the side of your case exactly how you want it. I've never painted a case before but I know that heavy coats of paint cause dripping. and you definitely dont want that. So once you got your template on there and taped, go ahead and spray in a quick coat. your gonna want to put a coat on at least 3-4 times so that is why the template stays on and taped. Once its dried go ahead and remove the template. It look good? Well i hope it does. re-post with some pictures and maybe some advice and adventures you had while painting a decal on your case.
Just cut out a template you made on thick paper that wont bleed. Then tape it how you want it and paint the inside of the template with multiple light coats of some cheap car spray. Then when its dry, take the template off and Wa-la!
I'm sure there is a more "professional" way to do it. But that is how I would probably do it. And I say car paint because I know that's what most people use to paint their cases.