Cooking Help Needed

As I've posted here before, I tend to pop my own popcorn on the stove. None of the microwave stuff for me. So, while reading up on things to make in my new cast iron dutch oven I came across some recipes for making popcorn in it. The all seem to consist of a stick of margarine, and then enough popcorn to cover the bottom of the pot. My first try was a disaster. One thing that seemed to take forever was for the margarine to heat up. I'm not sure if I should have it hotter to start with or what. I can just imagine if the margarine was burnt it not tasting that great. So has anyone else made popcorn in a dutch oven before? How hot should it be? Any other recommendations? I'm sure a level of it is practice. I know that in the usual pot I use to make popcorn I have it down to a science, it comes out perfect almost every time.