I started by luring him in position to put his head on the palm of my hand. I then lowered my hand in increments until it was touching the floor. Then I phased out my hand until I could just point to the floor. After a while I transferred it to a verbal command.
In the same way River learned to put his chin on tables, window sills, my lap, arms of chairs, etc.
I hope my explanation made sense. Do you do a lot of training?