With a broke horse, they usuall know steady pressure or see sawing the bit as a cue to back. Which ever method your horse knows is where you will start. Give them the cue to back and roll your back at the same time. I usually tell my students to roll their backs, like you are creating a "C" with your torso. I have found that when I say lean back, they have a tendency to lean to far and really put pressure on the bit. Sometimes, even horses that back with bit pressure don't need a lot.
Occasionally, when you lean back the horse may be confused for a moment and not back up. Don't pull harder, but wait for the horse to figure it out. When you get that one step, sit up straight and release the bit at the same time. Pet your horse, or use whatever your form of reward is. As with anything, you will continue this process and as the horse understands you will ask for more and more steps.
Once the horse is backing easily and understands, you can start to add bumping your legs or wiggling your feet (or what ever cue you decide you want to use) to ask for speed backwards.
By this time the horse is going to be getting pretty used to your weight shifting back and the horse backing. That is when I will start to move to the seat cue first. I don't expect the horse to do it right off the bat (but it feels pretty good when they do
). I will lean back, give the horse a couple of seconds to back, then start gently bumping my heels. If the horse does not back up, or tries to move forward, I will pick up the bit to reenforce the back cue. I continue this pattern until the horse learns to respond to the seat cue on its on.