You need two people.
Bowler repeatedly throws the ball at a point on the popping crease to the off-side of the batsman, to simulate a wide, over-pitched delivery.
Batsman is the one learning. He must:
1. Keep his eyes level at all times.
2. As the bowler raises his arm to throw,
(a) raise the bat to a high backlift
(b) step across with his back foot to put it in line with the ball
3. Step forward and play the ball with a straight bat, driving it gently to the off side.
Repeat the exercise until the bowler is too bored to continue, or until Jacob drives the ball straight back at the bowler from six feet away, striking him sharply on the elbow and making him cry like a girl.
The point of the lesson:
We want to get the batsman's feet moving, and make this sequence an automatic reaction to an offside ball.
Putting the back foot into line with the ball makes it possible to play with a straight bat, instead of swiping.
Stepping forward means that the ball is played down, and that the bat is moving faster when the bat strikes the ball. Cricketing types say that the "batsman's weight is behind the ball", and that there is a feeling of "leaning into" the shot.