First of all, not all air poppers are suitable for use as a coffee bean roaster. Looking down into the barrel of the machine, you should see that the hot air enters the chamber from side vents, NOT from a grate in the bottom. Chaff can accumulate on the bottom and it could be a fire hazard.
Alright. You have your green beans, and you're ready to try your first batch. Make sure you're working in a well-lit and well-ventilated area. You'll need to be able to watch the beans to determine when they are done to your liking, and there may be smoke and dust flying around during the process.
Use as many beans as you would corn kernels, anywhere from a half cup to a cup, depending on the popper. Don't overload the popper. Besides, you don't want to roast more coffee than you can use if a few days. Start up the popper, and make sure to have a bowl under the spout to catch dust and chaff. After around 3 minutes, you'll hear the first crack. You'll have a nice light roast in a minute after that, or up to another 3 minutes for darker roasts. Much longer than 6 minutes total and you'll have burned beans. Watch your beans through the clear top of the popper to determine when they are dark enough for you. It will take a bit of practice to get to know the right shade.
Pour your hot, roasted beans out into a metal colander and stir them around a bit to help cool. Voila, you now have your own freshly roasted coffee beans. All you need is to grind and brew. You'll never go back to pre-roasted coffee again. Store your beans as you would any bought beans.
Once I get myself some green beans, I'll be trying this method myself. I'll let you know how it turns out.