This isn't a real goal for me, as I don't have a trailer to practice on.
When I was teaching my mare to trailer, I would lunge her outside the trailer, and if she stepped on the ramp, she would get to relax. She learned really quickly that trailer meant rest. She now trailers no problem. A little iffy going home, but no rearing and pulling.
How would I teach her to self load if I wanted to?
A few times while I was lunging her, she would look like she would just hop into the trailer, but she never did.