First step is really to look at bikes from the same contract and see what style of lettering they use. The Axholme stencils can be used to give a (fairly) convincing facsimile of a sign-written bike if the paint is carefully brushed using the stencil as a guide.
I've had some help from friends in the past and recently I tried myself for the 2nd time and I'm happy with the result.
You have to look first what font you like the best. Than you have to copy it and make the combination of numbers (and letters) you want. Print it out and chalk the backside of your print (the sector where your numbers are). Than hold the paper on your tank (or front fender) and use a pencile to overwrite the lines of your print. It will look like this:
Than you can either tape the edges (if you want to spray), or just paint the inside of the lines. I painted it:
Wait for 24 hours and than remove the chalk. The endresult looks like this:
Don't be afraid to try. I have the shakiest hands in the world so anyone can do this.