The smile is a simple trick that is great to use when you are first starting clicker training. It helps establish the association of the click with a reward, and it helps build the horse's (and your!) confidence with clicker training. Initially, reward any attempt to lift the lip, even if it is to grab at the treat in your hand. Then, once they are consistently lifting their nose, you can focus on keeping them out of your space and not being grabby. Of course, if your horse is being aggressive, you will want to correct from the beginning so you don't teach bad habits.
In this video, I am working with a horse I have never worked with before and who has never done clicker training before. He is a bit grabby, so you can see how I dealt with his behavior. I also attracted quite a crowd with my treats, so enjoy their cuteness!
To start this trick, hold you treat in one hand and touch or tickle the tip of the horse's nose with that hand. When they make any effort, click and reward. As they start to get it, only give a treat every other time, and reward only good efforts. You can also start cuing by just waving a finger or your hand without touching them. Once they get it, try moving a little further away from them and cuing, and try to get a smile without them taking a step towards you.
I tend to give bigger rewards and pets for bigger efforts, to encourage motivation and participation. This guy was very motivated already, so I had to make him back up and out of my space several times. This whole session probably lasted 15 minutes, and the next day I went out and he smiled when I waved at him!