It's not so much an exercise as a test. If you have correct vertical alignment, that is, a straight line from ear to shoulder to hip to heel, then it will be easy. If you find yourself falling backwards or forwards your alignment is off. Correct your alignment by getting your seat over your feet and sitting up straight, then try again. It's a way of evaluating your position and balance without needing a groundperson or mirrors.
When I was working for an eventer in Maryland and we had "workout days" double posted trot was our break. The rest of the time it was two point without hands, rider's push ups, two point with one stirrup (much harder than two-point without stirrups!), and other fun, muscle soreness inducing things.
Double posting comes naturally to me now because this is how I change my diagnol to keep off my horse's back. Just keep trying, it was really hard to get the hang of when my trainer told me to start doing this when I change my diagnol when I school.