Take a big pan, put your oil in it and heat on medium. Add a couple of kernels ONLY in the oil and cover your pan. When the couple of kernels POP, open the pan and add all the remaining kernels plus your salt in an even layer. Cover and remove the pan from the heat, then count 30 seconds.
(This method first heats the oil to the right temperature, then waiting 30 seconds brings all of the other kernels to a near-popping temperature so that when they are put back on the heat, they all pop at about the same time)
Return the pan to the heat. The popcorn should begin popping soon, and all at once. Once the popping starts getting faster, gently shake the pan by moving it back and forth over the burner. Try to keep the lid slightly ajar to let the steam from the popcorn release (the popcorn will be drier and crisper). Once the popping slows to several seconds between pops, remove the pan from the heat, remove the lid, and dump the popcorn immediately into a wide bowl.
With this technique, nearly all of the kernels pop, and nothing burns.
If you are adding butter, you can easily melt it by placing the butter in the now empty, but hot pan then drizzle it over your popcorn.