This is one of the most often used roasting methods at home. All you need is an air popcorn machine with at least 1200W power. Put approximately 50gr of coffee inside the popcorn machine and switch it on. The weight of the coffee beans must be low enough so that the air is capable of stirring them adequately. If you use a lot of coffee beans then the air will fan air will not be able to stir the beans, and you'll get an uneven roast. If you use very few beans then the hot air will pass through the beans without any difficulty and the heat will not get trapped inside the roasting chamber. There is a chance that you may have to help the beans stirring at first with some self-assisted stirring.

Gradually as the beans roast, they lose water and they get lighter. The lighter they get the more easily they stir. So in other words, experiment, find the optimum roast batch size and then stick with it.

Some popcorn machines are not powerful enough to roast coffee without any modification. If you have such a machine, then you will have to make a small modification. Open the machine (make sure it's disconnected from the electric plug!) and then unscrew the roasting chamber. You will notice that apart from the heating elements there is also a small fuse and a thermostat. Bypass the thermostat and the fuse so that the machine is able to reach higher temperatures. It's very important to be very careful when you make these changes! Put everything back together and test if the machine works correctly. Put 50gr of beans and switch it on. Never leave the machine unattended! When the beans reach sufficient temperature you will hear the first crack (this will take less than 8 minutes). If you leave the beans roasting they will reach 2nd crack in a couple of minutes. Stop the roasting process when the coffee reaches your desired roasting level. Pour the beans in a colander, shake them and blow some air with your mouth to speed the cooling process.

There are a lot of people who have written analytical information how to modify a popcorn machine for roasting, and how to improve control by separating the heating and fun circuits, and adding variable resistances. If you want to find more information search for popcorn roasting in a search machine of your choice.