How much dressage education do you have?
Its difficult to explain exercises over the internet as it really depends on the rider's experience.
But basically, lots and lots of circles, turns and curved lines. Avoid straight lines, they encourage a horse to stiffen and fall on the forehand unless 100% engaged.
Leg yield and shoulder in/shoulder fore are brilliant exercises for opening the shoulder and releasing the back. Also multiple, correctly ridden transitions. Don't ride more than 12 strides at a time in the same gait/tempo. Trot-canter-trot transitions are my favourite for releasing a horse's back, they literally HAVE to use their back in their transitions.