The first one I did was the "Tweak-O" kit from Small Bear Electronics. It came as a kit with everything you needed except for a soldering iron and solder. The kit was around $40 and had really complete detailed instructions on the site designed for a beginner. Here are a few links to the instructions and to the actual kit.
Build The Tweak
Kit - The Tweak-O
Prices pretty much tend to go up from there. Some kits are more expensive than just buying the pedal to begin with, but if you are like me, you find it pretty cool to be using something you made yourself.