"Shake" is one of the first tricks I teach my horses and it is very easy. Also, it sets the stage for many other tricks like counting, Spanish walk, bowing, and pretending to limp.
I take a stick and gently tap the hoof, tap tap tap tap, until the horse lifts the foot. Then a treat and stop. I do that about 3 times a day. It doesn't take long before they lift the hoof when they see the stick pointing to the hoof. Then, they don't get a treat until they extend the hoof out a bit . . . which they will do when they don't get the expected treat. And then a little more, and a little more each day until they are gently thrusting their hoof out for you to shake.
Be aware that not all horses are interested in doing tricks. I have read (and found it to be true, personally) that a good trick horse is going to be mouthy. You want an equid (donkey or mule or zebra as well as horse) that enjoys people and wants to do things with and for them. It took me FOREVER to teach Isabeau to shake and she did it with great disdain and disgust. I would tap her foot a hundred times and she would literally roll her eyes and sigh. She clearly thought it was stuuuuupid. Chorro, on the other hand, couldn't wait to do tricks. "What are we going to learn today, Mom, huh? huh?"
Don't beat yourself up if your equid doesn't want to do tricks. Some love it. Some hate it. Remember it's like jumping or working cattle. Some animals can't wait and some would be happy to never do it.
By the way, I teach the horse to differentiate between "shake", "count", "bow", and "limp" by where I put my hand (because soon you can dispense with the stick and just point your finger), and also a verbal command. Some horses respond perfectly to just verbal commands and other ones only go by the way you hold your body.
If you are enjoying tricks, "hug", "kiss", and drinking from a bottle are also very simple easy tricks that horses can learn in just a day or two.