First find a steady, quiet , reliable horse.
Second find nice knowledgeable friend to lunge and control pace of horse from centre of arena.
Adjust stirrups carefully so that you are able to support foot in stirrup at slight angle with toes pointing upwards
Mount up, relax but maintain an upright posture.
All you have to do now is to sense the rythm of the horse.
Hold reins loose - no jabbing the mouth of horse just because you lose your balance.
The shock of the horse's up and down movement will be partially suppressed by the joints in the toes, the ankles , the knees and the under thigh muscles.
Sit wide with legs off from the flanks of horse. The disaster is to grip with the knees.
Whilst much of your body weight will rest on the bones of your bottom, the upward jolt from the horse will be suppressed eventually by the muscles of the under thighs as and when they have developed.
Imagine that you are crouching on the floor when the floor is moving up and down. The upward forces will be absorbed largely by the bend of the knees together with the other joints in the ankle,the foot. The hips will also take up part of the thrust. The aim of the sitting trot is to keep the posterior flat on the seat of the saddle, regardless of the upward thrusts.
If the horse speeds up then the exercise becomes much more difficult.
The sitting trot is best practised on a horse that will jog - ie slow trot and not all horses will adopt this pace willingly.
Do practice but only on a fit well muscled horse and for short distances only, your sitting trot whilst going uphill on a gentle slope preferably on a tarmacced surface.
Alternatively practice on a friend's Western saddle with a sprung seat.
Those spectacular, long legged, well schooled German horses are bred carefully, partly to do this movement with style - it takes years of practice to achieve this movement with panache.