I used stitching tape from a fabric store to make the stencil for my new stripes. The tape is just like regular masking tape, but only 1/4" wide. I think quilters use it.
If you're starting with a Don Post vinyl helmet, it's best to sand the helmet, apply a grey primer, and start your paint job from scratch, adding the stripes near the end of the process.
Someone out there will correct me if I'm mistaken, but the ESB helmet has 14 yellow stripes, and the ROTJ helmet has 18 reddish-orange, I believe.
When I did my first helmet, I used a Wal-Mart paint color, 'Catepillar Yellow'. However, the name (probably due to copyright problems) has been changed to 'Equipment Yellow.'
There are several other colors available on the market that will work just as well.
Also, there are stickers available that do all the work for you! Just peel and stick them to the helmet.
Hope that helps a little...