Sit the trot. And feel where your hips are moving. Bare with me if your outside hip is down the outside front leg is out. I think that's it. So what I do is sit two beat bump bump if my inside hip is down I want to post on the next beat. My instructor wants is to watch the shoulder and leg but that's difficult when the horse stops troting when you lean forward. So I had to learn to feel the beat. Once this is second nature is is easier to post on beat and off in two strides.