This is a great way to make popcorn. Especially cool is the way you get un-popped kernels out of the bag.

Put the popcorn kernels in the bag.
Fold over the top of the bag 3 or 4 times. I usually do it a couple of times in one direction, and then a couple backwards.
If you have a popcorn setting on your microwave, use that.
Otherwise, set your timer for 2:30.
BUT WATCH IT CAREFULLY!!! 
Once there is about three seconds consistently between pops, it is time to pull it out.
My microwave popcorn setting is 2:24, but I usually pull the bag out about 20 seconds before the end.
BUT WATCH IT CAREFULLY!!!

Especially the first few times you do this, so you don't burn your house down!
Now, unfold the top of the bag, but DON'T pour out the popcorn.
Cut one corner of the bottom of the bag, making a hole big enough for an un-popped kernel to fall through, but small enough that the popped ones can't escape.
Shake the bag over a garbage can with that corner down.
The reason you unfolded the bag is so that the popcorn can move around more freely and allow the "grannies" to work their way down and out.
NOW you can dump the popcorn into a bowl and do whatever you do with it.

If you can't handle this from here, you probably shouldn't be using a microwave.