I had to teach my 4 yr old this, as well. He knew the vocal command "trot" from lunging, but didn't get the concept in head. I used a long lead rope, and while jogging forward I would give the trot command and swing the lead rope behind me to tap his butt. Be aware he might not get it at first! My gelding first jumped sideways. With repeated attempts in a row, he got the idea that the command plus me jogging equals him trotting. Once he starting trotting - lots of praise! Took about 5 times of this and he nailed it.