When teaching a horse to be comfortable around their head, you need to remember two principles of natural horsemanship:

1) The reward is in the release: This means that when you are touching the horse, you
need to make sure you only stop touching when the horse is being
pleasant. If the horse were to toss his head about and you
stop touching him, then you are releasing the touch, and therefore
rewarding the horse for tossing his head. It is important to keep
touching until the horse is still again. Only reward behaviour you
want by only releasing touch (pressure) when your is behaving the way
you want him to.

2) Encounter, Wait and Revisit: This means that when you approach the horse with
something scary or uncomfortable and the horse starts tossing his head,
you need to wait and keep the same pressure/distance/touch
until the horse is calm and pleasant; once he is calm and pleasant, you
can take the object/pressure away from him and revisit (try again).
This is because if you show the horse that you recognize he
is nervous, but that you will only release/remove pressure when he is
still and calm. You also prove to your horse that when he is calm and
quiet, you will take the pressure away and start from the
beginning (instead of quickly advancing).

Putting the principles into practice: