I work on this in the stall, all I do is stand next to my mare like I was haltering.
I don't use a halter or lead rope I put my hand on the poll and apply the pressure and once she lowers her neck I release and repeat. I do this all the time if were just standing there chillin I have her lower her head or in ground work, cross ties...ect.
Then with the Haltering since she all ready knows how to lower her head. I start to put the halter on (strap on poll) and instead of having me put it on all they way I wait untill she searches for it or puts her noes in the halter. If she dosent put her nose in it right away since my hand is already holding on to the strap over the poll I can ask for the drop. But ofcouse you have to hold the halter open so your horse can actually putt his nose into the halter. That's how I do it, hope it can help.
We had this one old gelding he would run up to you and nose around the halter till he got it on, he was some times quite pushy if you didnt have the halter all ready!