This will sound silly, but here's how I learned it and always taught it. ( I was 16 when I learned) The front wheels are the only ones that do the work. Watch the tail of the trailer in the mirrors, but think of the front wheels in your head. Think of them as backward arrows. Nudge the trailer in either direction using the backward arrows. The more you point them them the "faster" the trailer turns. When you have the trailer turning at the desired rate, unwind the steering wheel and follow the circle you created. I never open the door, and always use the mirrors as long as I can see the tail of the trailer in them, then look over my shoulder. Always try to back to the left, in your left mirror, and always use a spotter in new places.
Find a huge parking lot and see if you can back the rig in a complete circle by "nudging the arrows" and "following the circle."