KISS
This trick is just a gimmick and it should only be taught to the most gentle of horses, that you have built up a real connection with.
I can not stress enough that It is very dangerous to allow your horse anywhere near your face, especially with his mouth.
If you try this trick and your horse grabs or is rough, do not continue.
I have found that this trick does increase the connection with your horse though, and builds up trust and a gentleness.
For this trick I use small pieces of carrot as treats, as pony nuts are too small.
Make a kissing noise and put your treat near your mouth.
When your horse gently uses his lips on your mouth, give him his treat.
Continue practising making the noise then treating when he muzzles your mouth moving your hand further and further away.
Final Thoughts
As I have said this trick is only a gimmick and not to be done if the horse is rough or grabby in any way.