The concept and design are really actually simple. You get a small generator from a rechargeable flashlight, rig it up so you can turn the generator axle when you step down, and channel that energy to a cell phone's (or other device's) charger cord. You can use this when you are jogging or walking or just when you're sitting down and feel like tapping your foot.
The materials are really pretty cheap and the project is easy to make. The only thing that you will have to adjust for is that you probable won't have the same kind of shoe I have. So you might need to make some adjustments, but the concept is still the same.
Step 1: Materials
Next, you'll need two rechargeable flashlights for their generators. The two pictured below are the two I used and the next picture is one of their generators. I got the flashlights from my local Radio Shack and the brand is MegaBrite.
The other materials are: a spring, some wood, a small piece of re-bar, some thick wire, some small screws, and a cell phone charger.
As far as tools go, you'll need some basic hand tools, a drill, a band saw (unless you want to do it by hand), and some electrical tape.