Just start by doing click and then treat, click and treat, click and treat. Bites of their daily grain works great with horses, or bits of carrots, apples, or horse treats. It will take a few repetitions (usually 20-30) for them to hear the click and look for a treat. Then start training with it. Something really easy. I usually would do something like a nose touch (as in touch an object or my hand with your nose. It's such a simple behavior, and if you stick something up in front of them, a lot of animals will reach out and investigate it which means you can start rewarding right away.
I'd read up on clicker theory so you understand fully how to do it before you start.