A friend of ours does a lot of work with mustang and kill pen weanlings and yearlings, many of which arrive at his place scared and wild as deer. This is how he teaches them to be haltered; it's low stress, safe, and it works. Of course, if your foal is wild and scared, you take more time getting up to them, but after you can walk up and touch them and rub on them without them leaving, this is what he does:
Put her in a stall or small corral, and get her used to you walking up and putting a neck rope on her-- an old leather belt or stirrup leather works well for this. It's easier to get on than a halter as it doesn't go over the face, and when properly placed, loosely up behind the ears, you still have some control but it won't pull tight like a lariat rope will. Put it on, pet her, take it off. Repeat until she stands quietly when you approach, isn't bothered when you put your arm over her neck for it, and doesn't take off when it's removed. It may take a few days. When she's comfortable with that neck strap, clip a short lead rope to it.
Then put it on, lead her with it (a step or two is fine, just enough for her to learn that pressure on it gets her to move her feet) and when you ask her to stop, place a hand on her nose, too. Then rub on her, including her face and head until she's good with that. Remove the neck strap and you're done for the day. Repeat a few days.
Next, put on the neck strap and then rub her all over with the halter, including her face and head until she's good with that. Then open the halter and put it over her nose and back off, over the nose and back off, over the nose and back off. If she raises her head or turns her nose away, simply stop with the halter right there until she moves a fraction of an inch back toward you or down, then step away. Teach her that lowering her head and waiting for the halter is what you want. If you only get it to touch her nose the first day, fine. You have time. Once you can get it over her muzzle and back off, be done. The next day, put it on and off, on and off over her nose. Don't rush to buckle that halter; the time you take here will pay off the rest of that baby's life. If she moves away, you have the neck strap and she knows how to stop with that, but she likely won't move if you do this right.
After a day or two, then put the halter on and off, on and off, on and off. If she's comfortable with your arm over her neck to pull on the halter, as she should be, she really can't pull away, and you can control her if she does. Make that such a normal part of her life, she accepts it like she does the water tank, hay feeder, and barn dog. Start in the small pen or stall so she can't run off too far, then graduate to a larger area. The idea here is not to go to a larger space until you're 100% sure she's fine with you haltering her, so she doesn't learn she can pull away. Keep using the neck strap if you need to to ensure this. Go halter her whenever you think of it, then rub on her or occasionally offer an handful of grain, unhalter her, and leave. Let her associate being haltered with good things. If you need to catch her for something unpleasant, do ten good halterings after that. Go catch and halter her before you lead her in and let there be a handful of yummy feed or a treat in her feeder. Catch her and scratch her favorite spot, then let her go. Make her think that halter and you approaching with it is a good thing.
Within a week or two, your foal should be very easy to catch and halter. And because you used the neck strap, simply putting the lead rope or arm over her neck will mean she stands quietly for haltering or bridling, too, and she won't turn into a ten foot giraffe or try to back or duck out of a halter. Good luck with your baby!
*don't ever leave a neck strap or halter on a foal without you being there. Make sure that you're in a place where you can get up to her to take it off, too, even if she does pull away at first.