You ask the horse to continue rocking back farther and farther until the knee touches the ground, always allowing the horse to come right back up. You will want to keep the nose pointed toward the shoulder away from you, not toward you as he will lay down the wrong direction and not between his legs as he will not be able to lay down as easily. When they are comfortable with that I start asking them to keep the knee on the ground a little longer before asking them to get up. It is here you start teaching the cue to get up so the horse will stay down until you give the cue to get up.
When the horse is leaving the knee on the ground I will change the rope. I will run it under the belly to the opposite side, over the back and back to my hand. This will help keep the foot up if the horse resists the next steps. With the horses knee on the ground you will take his nose to the opposite side that you are standing on and ask the horse to rock back more. He should slowly come down on his shoulder and then lay down. Let him up as soon as he wants.
If you have not already, start using your cue to lay down. Lay him down a couple times letting him get up, then you will ask him to stay down by keeping his head bent to his side. Increase the amount of time he stays down and be sure you are giving a cue to get up. When your horse learns the cue to lay down he will do it in a natural way, so start asking with the cue first and if he does not lay down, do it manually. Rub him and get him comfortable with you being around him while he is down. Don't scare him. As soon as he learns he won't get hurt, he will lay down with ease.