I've done this before! It was a long time ago though. I'm pretty sure I just put the kernels in the lunchbag, folded it over and microwaved it for a few minutes until there was a few seconds between kernels popping. I'm not sure if I sealed the top of the bag though. I'll defer to mala.
Yep! Half an oz to one oz of kernels in a brown lunch bag. Fold the top over twice, just half an inch, to keep it closed. Lay it in the microwave on its side and set it for 2 min. Stay by the microwave ans as soon as the popping slows down to 2-3 seconds between pops take it out. Top as you wish. No nasty fake butter, no tons of salt, and it doesn't make more than I can eat in a sitting.