always remember to release as soon as you feel them give AT ALL to pressure. I like to start my horse on a circle, keeping contact with the outside rein and playing with the inside rein until I feel a give. No matter how little the give is, I immediately reward by taking away the pressure. as soon as the horse comes out of the frame, I start playing with the inside rein again. once the horse understands what you want by the fact that you let pressure off every time he gives with his mouth/head then he will start to carry himself. Remember that if the horse is not used to carrying himself, he might only be able to be on the bit for a couple of steps- thats ok! reward him and don't over-do it until you know he has built up the musculature needed to carry himself for longer periods of time. set goals like, today I want him to carry himself on the bit for one circle and when you meet that goal, reward him by stopping for the day. The secret for me is giving the horse reward for positive behavior so that he understands what is being asked and not making unrealistic goals and being patient! it is easier for a horse to carry himself on a circle than a straight line
Massage your reins gently as you ask for more forward at the trot. Control your pace with your post so your horse does not get to fast. Keep massaging and asking for forward until your reins are just about to the buckle. Make sure you keep the contact...sometimes it takes a while to get them to really stretch and it may take a couple of times! Be happy with slow progress and reward with the right behaviour!
Work on lots of transitions and circles, leg yields, spirals, basically anything to get the haunch moving and work on a loose rein more to allow your horse to stretch down and find the correct posture. It's kinda something they have to experiment with and understand. You could also look up videos on YouTube I know they have some good ones with professional trainers. You could also ask your trainer for tips if you work with one