Get a stopwatch, a colander, an oven mitten, and a wooden spoon.
Measure 2/3 cup of green coffee beans.
Plug air popcorn popper into outlet or extension cord.
Add the 2/3 cup of beans.
Start the timer.
Cover your stirring hand with the oven mitt.
Hold the spoon by the shovel end and stick the handle end into the beans and start stirring vigorously and stir fast until finished roasting.
Use the timer as a guide. After about 40 to 50 seconds, the papery outer husks fly up in a large quantity. Some husks will start flying up a little earlier.
At around the 2:40 to 2:50 mark, rapid cracking begins. This is first crack. Some cracking might start earlier like one crack occasionally. But multiple cracking starts around 2:40.
First crack continues for about a minute. When it starts to slow down, you can consider stopping. Watch your bean color change from olive green to light brown to a warm brown and to a cool. dark brown. Stop when you like the color.
For the first batch, I stop by 3:45. For the second batch, maybe because the popper is still warm, the start times for the husks flying off and for first crack begin about 15 to 30 seconds sooner. That's why I use a timer as a guide. But watch and listen to the beans too. For the second batch, I stop between 3:15 and 3:30. For the first or second batch, some beans will still be cracking during first crack when you stop. Waiting for first crack to end completely will make for a darker roast. Of course, roasting until and through second crack creates the French roast or dark roast. Cold weather update: When outdoor temps are in the 30s or colder, timings are about 40 seconds longer. I stop roasting around the 4:30 mark.
When the end time is near, unplug the popper and dump beans into colander.
Cool beans by shaking them in colander and use your hand that is covered by the oven mitt to stir the beans too.
When beans are cool enough to grab with a bare hand, then spread beans onto a plate to rest overnight.
More info at Roasting Coffee Beans at Home