OHHHHH!!!!! I know this one!!! LOL! Okay, first thing I would suggest is getting a DOUBLE jointed snaffle like a KK snaffle or the JP by Korsteel Oval Mouth Copper Loose Ring snaffle. Reason being is that if you want to get the horse soft, a double jointed bit won't hit the roof of the horse's mouth when you use 2 reins, and it wont' collapse on the bars. Using a single jointed snaffle in the following exercise WILL NOT WORK. It'll just make your horse uncomfortable. :) Oh, and I use the JP bit b/c that's what Parelli suggests for this exercise.
Okay, so what you want to do is start at a halt. Start to comb the reins through your hands, in a hand-over-hand motion and let them slide through your hands. What you want her to do is reach for the bit. The more she resists, or if she doesn't reach with a soft combing feel, the more drag you put in the reins. When she reaches, release the reins completely and rub her. Then progress to the walk, and then the trot. DO NOT do this at the canter! When you are combing the reins, you want to be really fluid in your upper body, really move your shoulders and suck your sternum in. After she gets the idea, when she reaches for the bit still keep a nice, soft, flowing feel on the rein you have left. Let her stretch as low and for as long as she wants to.
Did that make sense? :)