For sitting trot- the best way I learnt how to sit was with no stirrups or reins (you can get someone to put your horse on a lunge rein and do this). But remember to sit up tall, don’t tense your body as your pelvis should move with the horse. Try to put Your weight on your heels. Riding bareback helped a lot as well.
For posting - keep weight in heels, and rise with your thighs. I was taught to rise when the outside leg touched the ground .
Wish you all the best!
I would recommend getting a good solid sitting trot foundation because moving onto the canter!! I stayed trotting for, goodness months, until I went onto to canter because absorbing the bounce from the sitting trot helps you move your hips and create that rhythm in the canter.
Keep in mind I’m more of a peer and I’m not a trainer ...
Make sure you’re squeezing your lower leg against your horses sides, and when you post and stand up make sure you’re putting weight down in your heels and the balls of your feet where your stirrup irons are. Hope this helps you, and good luck with your pony! I’m always up to chat if there’s anything else that I’m able to help with .
For the first couple of times look down at one of the horses front legs and when it rises you rises when it lowers you lower and grip with your calls and heels down will help with you balance eventually you'll get the ryhme and you won't have to look down