These is no short way to explain how I make my own, so here it goes. Find some Testors "white" decal paper. You will need to also get there decal bonder spray.
Get on your computer, open a word document. Find the found you want, type the number you want then play with the font size until you think you have it right. Type the same number several time with different font size. Print this out, then cut them out and try them out for the correct size. Once you have it, save that document to your desktop.
Now go to file, page set up and then find the tab at the top of that box called paper. Once you have that open, adjust the width and height to the decal paper size. Now cut down a sheet of printer paper to that size and finger out how to feed it into your printer.
Once you have this figured out, print again with the white decal paper. Follow instructions for bonding it. Get MicroSol and MicroSet for applying decals.