I point the whip to my horses rib cage/elbow area. And then slowly increase pressure like slightly waving the whip, then touching him and then walking towards him and then eventually chasing him out. It would take time for them to get the hang of it and develop a habit of it but this is a good start.
Remember to use your body language too. Broaden your shoulders, stand tall. And look directly at where you are pointing the whip to push him out. You gotta do it with purpose.
To be honest it’s more of a waiting process, it also depends on if you use a bridle when lunging or a head collor or proper lunging stuff, bridles are very helpful but it ranges from horse to horse