Firstly, you have to understand why you have to post while totting on a Horse. This makes things more comfortable for the rider and is easier on the horse's back.
Then, you have to understand diagonals. During a trot, the horse's left hind leg and right front leg (which are diagonal to one another) move simultaneously, and vice versa. At first, it can be difficult to tell which diagonal your horse is on. A good trick is to watch your horse's outside shoulder. You should rise out of your seat as his shoulder moves forward and sit back down as it moves back.
Gather the reins so you're in contact with the horse's mouth. Don't pull back, rather act like your elbows are removed from your moving body and you're holding the horse's hand (or, rather, his mouth).Post forward and up simultaneously, then down and back. As you rise your pelvis should go forward and up, gliding up into the space between your hands without any interruptions. Your back should be straight, with your chest up and your shoulders back. As you go down, sit softly into the saddle only for enough time as takes to rise again. The entire movement of posting should be very upward based, and when your seat touches the saddle it should be done very lightly, and immediately be proceeded by rising again.
Of course, the best way to learn is to try 😉
Good luck !