If the horse doesn't understand the whip then you can take his/her leg and slightly pull it towards the stomach while giving a command
Thank you very much Ines for your help!
Hi Mathew! Put a halter and a lunge line on your horse. With the lunging whip touch his/her legs (do not hit the horse, just slightly touch) and say the word you want him/her to associate with the action. Then, if he/she lays down give him/her a treat and say good boy/girl. Do this whenever you can (even more than one time a day if you can) and I'm sure it will work out. This is how my trainer taught his horses.
Hope you succeed!