In order to post properly, you need to be aware of which diagonal your horse is on at the moment.
When a rider is posting at the trot, he is said to be on the right diagonal if he rises and sits out of the saddle when the horses’ right front leg rises and falls. Similarly, on the left diagonal. On-line Encyclopedia.
The easiest way to do this is to glance down at the horse’s shoulders. Don’t bend your neck to look down, try to keep your head up, level and steady. You will see one shoulder move forward, then the other etc. When the horse’s left shoulder “bulges” forward, the horse’s left front leg is forward. When the horse’s right shoulder “bulges” forward, the horse’s right front leg is forward.
The more you ride, the more you will be able to feel which diagonal the horse in on. This will become important later in this course.