Ok to start things off, you'll need to design your rubber band gun.
Start out with what material you want to make the gun out of and how to "cut" it. One way is to use laser cutting and cut acrylic sheets or even metal, however with metal it's a bit more expensive and limited for sheet thickness. Another solution to metal cutting is water jet cutting. This method they could use nearly any thickness of metal. For this instructable, however, will be using 1/8" thick acrylic sheets that will be laser cut from Ponoko
Next measurements. Measure out your hand so that you could design the gun so that it fits your hand. Then you could measure out how long you want the gun to be. Make sure that the rubber band you plan on working with are able to stretch long enough so that it could fit the gun without breaking.
Next is the actual design portion. This is often the hardest part of the whole process, I've even heard that rubber band gun designing can be even more difficult than actual gun designing.
To help start here are a few places where they make rubber band guns.
Site 1: OGG CRAFT http://oggcraft.jp/eng/eng_ratchet.html
This site is great. It has so many different, innovative, rubber band gun designs. The site link I've put on here is actually a list of they're different versions of ratcheting systems to deliver the rubber band. Release mechanism
: this link shows several of their different rubber band release mechanisms. Love coming to find inspiration for a rubber band gun.
Site 2: Over Boost!? http://yappaps3.blog.shinobi.jp/Entry/47/
This site actually became internet famous at one point for their crazy awesome full metal rubber band guns. The site link shows several different guns that they've created. If you click on, I think, any of the guns, there's is a small gif that shows the mechanism of the rubber band gun. They're often hard to see, and for good reason since the owner doesn't want to simply give their gun designs away. Personally took some time to figure out how the mechanisms work, but this site has the "classic" rubber band gun design with the rotating rubber band holder and releaser.
Sorry for the long rambling for each of the two sites, but these two sites really are awesome, definite first place I go to if I want to make a rubber band gun.
I'll talk about my design in the next step... this step is kinda gettin long... hehe