Step 1 is choosing a font that you like. I very much like a font called "City" that was designed in the 1930s for the Berthold type foundry in Berlin. So I chose that.
Step 2 is firing up Adobe Illustrator. Make a new document and write your desired numbers two times. “Create outlines” of the numbers (It's under "Type" in the top menu). This will turn your numbers into easily manipulated vector objects.
Step 3: select the effect called “Free distort” (Effects/Distort & transform/Free distort). You can now distort the numbers into pretty much any shape you like. In order for mine to fit in with the lines of my tail piece I made my numbers a little bit lower towards the end of the bike, which will emphasize the "leaning" effect. I added a black outline to my numbers for that "Nascar" look.
Step 4 is saving your document as an Illustrator eps-document and then emailing it to your local digital printer who will print them on vinyl and cut them out for you. There really isn't more to it!