This is how Xenophon would do it:
[I]In order to insert the bit correctly the groom should, in the first place, approach on the near side of the horse, and then throwing the reins over his head, let them drop loosely on the withers; raise the headstall in his right hand, and with his left present the bit. If the horse will take the bit, it is a simple business to adjust the strap of the headstall; but if he refuses to open his mouth, the groom must hold the bit against the teeth and at the same time insert the thumb of his left hand inside the horse’s jaws. Most horses will open their mouths to that operation. But if he still refuses, then the groom must press the lip against the tush (canine tooth); very few horses will refuse the bit, when that is done to them.
The groom can hardly be too much alive to the following points * * * if any work is to be done: in fact, so important is it that the horse should readily take his bit, that, to put it tersely, a horse that will not take it is good for nothing. Now, if the horse be bitted not only when he has work to do, but also when he is being taken to his food and when he is being led home from a ride, it would be no great marvel if he learnt to take the bit of his own accord, when first presented to him.[/I]
Of course FIRST the horse has to lower his head. I’ve found the easiest way was to cross tie them (loose enough so that they CAN lower their head) and then gently touch them on the poll and talk nicely to them until you get a small drop. Then generously reward them and do it again. For a horse who does not resist, just doean’t know what you want, this takes less than half an hour. Sometimes a LOT less. If the horse resists, particularly with a heads up move, then it will take longer. I’m not against using a treat to get the process started but if if you don’t need one then don’t use one. Save the treat for when you really DO need it! 
Begin by having the horse lower his head and follow Xenophon’s advice. Form your right hand into a backwards “c” and hang the headstall from your thumb. If you have a problem and your hand is open then you won’t be tempted to grab the headstall or get hung up in headstall. You pull your hand back and it will just fall off. If no problem then as Xenophon notes the job will be quickly done.
If you’ve never read Xenophon’s seminal work On Equitation then Google it and read it. It’s free from many sources. Then read his companion work, The Cavalry General. Most of the stuff published on most places is a rehash of what he wrote 2300 years ago!!!
Good luck in your project.
G.