You do it the old fashioned way. You physically show them. What I do with young ones is to put them in CF. I would stand behind them and grab a handful of jersey. I would have someone hit the ball to one side or the other. ( I cheated and pointed which direction I wanted it) I would break on the ball dragging the player with me. After a few times of being pulled around the outfield by the back of their jersey they figure out when, how hard, where to and how fast to get there. After a short time the girls were breaking harder then me and I couldn't keep up with them. After they are running at the ball you let go of jersey and let go after it on their own. My OF always got a good jump on the ball.
I dunno, dude. In this day and age you're asking fer trouble.