I would like to train my horse to lay down on command. Does anyone know how to go about doing that?

I have had her since the day she was born & she really imprinted on me. I can go up to her while she is laying down & lay down next to her or clean her hoofs, etc., but am now starting to break her. I have an old shoulder injury that prevents me from pulling myself up into the saddle & rather like the idea of getting on her while she is down rather than climbing on the bed of the truck.