How can I teach my horse how to salute?

If you horse is treat monster then she will catch on quickly. Assuming you and your horse already know a cue for picking up one of her front feet as if you were going to clean them you need to practice it where she does it with just a light touch on her chestnut or elbow. Each time she does it give her a treat. When you do give her a treat hold it a little lower each time below her nose until you are giving her a treat down near her knee thus causing her to bend her head down. Sooner or later she'll start picking up her foot when you start to reach down to touch her. At that stage start standing more to the front of her while you hold your hand up in a salute. Then after holding that position for 3 to 5 seconds reach down to her cue spot on her leg that you've been using for her to lift her leg. She will eventually start understanding that every time you salute her it will proceed you leaning down to have her pick up her feet which proceeds a treat. So she will figure it out that to get a treat faster she'll go ahead and pick up her foot as soon as she see you holding your hand up.

By the way don't try everything in one day. Take 2 or 3 days in teaching her and she'll be more willing to do it