I'm going to ask this now even tho I haven't done it with her yet. When I do start teaching her how to flex on the ground (or in saddle I guess) when I pull her head around to flex I almost know she is going to move to the side or back up. Should I just stay with her tell she gives me her head?
Yes, she might move around, especially at first. That's okay.
If she is resisting the bit, stay with her until she gives her head slightly. Don't worry about getting her to give her head completely at the beginning. You don't need to pull her nose to your foot. You just want her to begin to respond to the reins.
As soon as she gives a little, release the rein right away. This way she'll get lighter.
You'll be able to ask her to flex more as she gets the hang of it and loosens up. I personally don't pull their noses right to my foot, unless they're trying to buck me off or something.
As she progresses, if you find she's snatching her head away as soon as you release the reins, you can ask her to give, then release part way once she yields. She'll likely hit the end of the rein, but just hold your hand still and ask her to soften again. Then, once she softens, release all the way. Does that make sense?
Once she's flexing a little, you might want to get her to disengage her hip. So, ask her to flex her head (say to the left). Once she gives her head a little, you'll move your left leg back and cue her to move her hip to the right. She'll likely start moving, and once you feel her hind legs making big steps to the right, release everything. If you can't tell if she's doing what you want, have someone watch or do it from the ground first (I actually move the stirrup back with my hand and flap it into their sides, while keeping their heads flexed with a halter or bridle). This will kind of loosen her up and get her responding to leg cues.
I try not to overdo any of this. Once the horses are flexing nicely and disengaging their hips, I move onto other stuff (though I will do a quick refresher every now and then). If you drill them too much, I think they can get bored.