Warm up with lots and lots of circling, changing direction, serpentines, etc. Get her bending around the turns—inside leg and a little inside rein. This will get her moving and listening to you. Also, make sure she's moving forward off your leg and not just plodding along. Do this at the trot if you feel comfortable.
Once you feel she's thoroughly warmed up, go back to a walk but keep up a good steady pace. Then, lengthen your reins a bit and put your hands back behind your knees. Gently alternate squeezing your hands to feel her mouth with the bit. Soon, she will arch her neck a bit and lower her nose to go on the bit. Immediately soften and give her lots of verbal praise! You can give her pats too at first too.
This will take a lot of repetition at first, and you'll only be able to get it with your hands behind your knees first. But within a few rides you'll be able to give the cues from a regular position: leg on to keep her moving forward and alternating squeezes/pressure down the rein. I don't mean yanking—just take up the slack so you have a good feel of her mouth. When you guys are just starting out, I recommend you only do it at the walk. She'll only go on the bit for a step or two at first, but once she's got it for 4-5 steps without you having to put your hands behind your knees, you can start working at the trot!
Remember—always SOFTEN as soon as she goes on the bit. Don't drop the reins, just keep a good feel but lighten up and praise her!