"Frame" starts from the BACK (haunches), not the front (reins). Your trainer has it right in that your hands should be steady, but you should not be pulling the reins back so far.
Instead, make sure there's a nice bend to your elbow, your hands are UP, and your pinkie finger lines up with the horse's mouth (where you want it to be anyway, lol). Keeping your shoulders and arms relaxed, push your horse forward with your leg. Sit the trot, tuck your rear, let your legs hang down heavy/long, LOOK UP, and breathe. Pull your inside rein back a bit (2-3"), keep your outside rein stead, and just keep pushing him forward. If he rushes, less push with your leg (just squeeze, inside leg only), more with your seat. Just hold it and WAIT. As soon as you feel his head drop into the bridle, even if it's only 1/2", relax your reins (IMMEDIATELY) to a normal position (don't throw them away) and give him a "GOOD BOY". He needs to equate his "give" with your "release".
Take up your reins and try again. Repeat 4-5 times, take a break, then go the other way. Do this at the walk first, then try the trot. Quit on a good note. Let him "think" about your lesson, then try again the next day.
As he gets better at it, you should feel his back come under you and his hind end reach farther under his belly (your saddle will come up and you'll feel more "bump" in his gaits from the rear). Start asking him for long and longer periods with his head lowered. Be sure to keep him MARCHING forward. If you let him slow down, he'll topple on the forehand and you'll get false collection. Once he's doing well at the walk and trot, try it at the canter.
To help your progress along, start working on a medium circle and try counter bending work. First get a good marching trot going. Ask for him to give his head/neck. Once he does, slowly change the bend to the outside. Keep looking around your circle. Keep your hips turned in the direction you want to go. Keep your outside leg on him (HARD if you have to) to keep him on the circle. When you pull your outside rein back, keep it closed to his neck, so he is less likely to drift. Pick your outside rein UP a bit too, so it's back and up. Keep your inside rein steady (so he doesn't pop his shoulder) and keep looking forward and round your circle, LOOK UP! Feel his mouth (sit the trot). Keep asking with your seat and leg until you feel his head drop and mouth soften, even it's just 1/2". IMMEDIATELY relax your aids and go back to your regular (true) bend. Continue at the trot until he's relaxed and try again. Do this 4-5 times, relax a bit, then try it the other way.
Once he's going really well at the trot with the counter bending, try it at the canter, but on a BIGGER circle, and only AFTER he is doing VERY well at the trot. If you rush it, you risk him popping his shoulder, leaning, and learning other bad habits.