Jump in the direction of the enemy, then press jump again when you're next to it (but before you land).
Note: You have to press one of WASD to indicate which direction you want to kick.
The easiest way to do it is to start on the ground then quickly double tap next to the enemy.
It's possible to kick sideways, as well as forwards:
Did you find this confusing??? So does everyone else, which is why kicks are getting an overhaul for the next release. :)