Goal: Horse stands still and lowers his head while you place the halter on his head, halter is the only cue.
Stand in the same offset position as you have been but a step back. Have the horse target the halter a few times.
Then with the halter in your far hand, gather up the halter so that only the nose piece is accessible to the horse (essentially it will be inside out). Hold it from the top so it dangles down creating a loop for him to put his nose in with your hand holding it from the top of the loop. Shape him towards sticking his nose in the loop (under your hand). Click and treat him for targeting the halter with his nose, nudging it, sniffing through it and placing his nose in a little, then a little further.
Make sure to locate the loop at the height and distance from the horse where it would normally be when you cue him to lower his head to place the halter on (for most people that would be slightly above their hip).
When he is consistently placing his nose into the loop at that location, back your hand out and allow the loop to rest on his nose for a second, then lift and c/t. Rest the loop for longer and longer duration until you get to 20 seconds.
When you can rest the nose loop on his nose for 20 seconds, turn the halter back right side out, making sure the short side of the strings is on the side you are on. Use the far hand to hold the halter near the junction on the far side of the horse. Lift the shortest string on your side incrementally until it is at its highest point and the strap that is under his jaw is touching his skin. At this point your hand should be taking some of the weight of the halter and the horse accepting some and the string on the far side will be hanging down. Increase the duration of you holding it there, c/t as you go. Next wiggle it around a little so he gets used to the feeling on something under the jaw. C/t. Increase the duration of wiggle slowly to 10 seconds.
Next, hold the nose loop of the halter with your close hand behind his mouth and lift your far hand so you can raise the longer string on the backside in small steps until it rests just behind his poll and ears. C/t each step.
Next, allow the longer string to rest just behind his poll for a second, then lift and c/t and drop it back down to starting position. Each time, rest the loop on his head for longer and longer duration until you get to 20 seconds.
Now remove your close hand from the nose loop and use it to hold the shorter side of the side string up as if it was tied. Incrementally lift the longer string to where it rests on his poll.
Next, fiddle with the strings as if you are buckling or typing the halter up behind and below his ear. When he is comfortable with that, try actually securing the strings.
Next c/t for allowing you to adjust the fitting of the top of the halter, then the bottom in small steps until it is fitted properly.
Once he is comfortable with you putting the halter on completely, take it off and release him to go do whatever he wants to do between each practice. Freedom at this point is likely more rewarding than the halter. This process of pairing a more-desired behavior (from the horse’s perspective) with a less desired behavior is called the Premack Principle. It is also known as ‘Grandma’s Law’ as Grandma would have you do your homework before going to play, pairing a fun activity with a less fun activity.
Interesting Fun Fact: Over time, the less-desirable activity can become more fun for the horse that the one you initially paired it with!
At this point you are not leading the horse, just putting the halter on the horse. Allow him to wear it for increasing length of time while train other behaviors or work nearby, then remove it when unsupervised.
If you have a halter that attaches on the opposite side, practice with that starting from the beginning again so he gets comfortable with you working off both sides.