Start shaking the pan once you hear kernels starting to pop. Shaking the popcorn while cooking is important. It keeps everything evenly cooking, and also makes the kernels fall under the already popped corn, where they have a better chance of popping.
I found that taking a dish drying towel and putting it on top of the pan so that it covers both handles and the knob on the lid helps with this, because if you keep the towel tight enough you can lightly shake it without worrying if the lid is going to come off.
Be careful. There is hot oil in the pan and you don't want to get burned accidentally.
I don't recommend opening the pan while it's popping, or you might get popcorn all over your stove.