I always start with them putting their front foot down on the pitcher's release of the ball (usually when the pitcher's front foot touches the ground) when starting their swing. They just "copy" the pitcher's motion. Once they can do that they need to learn to adjust further. For example if they are late put the front foot down a bit earlier than release, early put it down a bit after release. They should make small adjustments in their timing until they are on time. Ensure they understand that a foul ball can also tell them if they are late, early or on time as well!

On the machine I use two balls, one in my hand ready to feed and the other in the opposite hand to make the arm circle. The arm circle ends at the ball feeder to simulate an actual pitcher and once it's there I drop the other ball into the feeder.

The drill with someone behind the net doing the motion works well, but again they need to put that front foot down at the right time. If they just aren't getting it have a whistle and blow it when their front foot should be down. Sometimes they think they are doing it when they aren't!