In addition to the great flatwork answers, I also send them into a small circle at the canter. This will make them slow down and rock back on their own. All you need to do is sit up and be there with your leg and seat (not sitting) and relax your lower back muscles to invite them to slow down, rock back, and take each step. It's very important to not let yourself rush them through the circle. If they break, it's okay and it gives you an idea of exactly how weak they are behind and how much you'll need to help them through the exercise. It's also important not to get tense in your hand. The best explanation I've heard for that is to hold the reins like you are holding a baby bird in your hand: secure enough that you will not drop them but not so tight that you'd crush them.
Adjust your position. By stretching upwards you bring your center go gravity to a smaller, easy to carry point which makes it easier for you horse to balance. Also encourage her to bring her withers upwards to encourage her to rock back on her hind end. If the rider is leaning forward at all it makes it difficult for the horse to learn to rock back also
Shoulder fore and eventually shoulder in. Concentrate on your inside leg and the horse's inside hind leg (you want the inside hind leg to reach for the outside fore) this is a great way to lighten the fore hand in addition to a lot of great exorcises listed by others. If you don't understand shoulder in - some help from a pro with dressage experience may be a good idea. Once you feel the changes shoulder in can make in horse (even a horse with down-hill conformation) you will go to this exorcise frequently. Good luck.
Learn to assist your horse through the transitions as opposed to possibly throwing them off balance - if you stop the horse suddenly , like a car or anything slowing down the weight will get pulled to the front and so your horse will go onto the forehand. Slow downwards and quick upwards transitions help get the horses stepping under. (paraphrasing my coach and living this rn)
Trotting poles, even some raised a little a great. Also believe it or not, this excercise is in the German school of training. Trot in a circle, at open side, halt, turn on the forehand ½ way (facing the other direction), then trot. And repeat. This helps their inside hind leg come under their belly. Then they can balance on their back legs under the rider and this helps the shoulder become lighter. Also, make sure you sit up and don't grip too much with the reins. Sometimes when I focus so hard on my riding, I get tense in my hands. I hope this helps!
Lots and lots of half halts. Remind her to sit up before she thinks about falling forward. Try half halting every few steps, to get her sitting back and stepping further underneath herself rather than out the front.
As Olivia said, transitions will also be very helpful. While working towards consistent engagement in the hind end, you can also focus on asking that before and during the transitions that you have her balanced and sitting up by half halting.
Lots and lots of transitions up and down all with the hind end engaged and the shoulders light. This is very difficult for a horse so it will not come immediately and will not be consistent until the horse develops the strength and flexibility required to carry and bend those joints. Also trot poles are awesome. You can also lunge in side reins