Jumping over stuff is fun! Especially if you are a JRT and you get to practice jumping over pillows. A word of caution, however, that I have had Gromette jump over another dog in the park many times successfully. But every once in a while, she misjudges and lands on the other dog.
Hold your leg or foot out in the air. When you are first teaching this trick, start very low to the ground.
Hold a treat over your leg and beckon your dog to get the treat. Use the command, "Over!"
As soon as your dog crosses over your leg or foot, give lots of praise, and reward your dog with the treat.
Do this a lot, and maybe start raising your foot a bit higher. Soon your dog will figure out that the treat is for jumping over your foot.
Once your dog figure this out, try teaching them to jump over other things, such as pillows, again offering lots of praise with every successful trick.