Just swing your butt up.
I'm not great at it, but I can get on bareback from the ground. I just don't look that great doing it!
Here's what I do. Stand facing your horse's butt a little in front of their shoulder. Put your left hand on their mane. Take a big step with your left foot, push off the ground when you step down (so you're hopping/jumping), and swing your right leg and arm over. I usually make it about 3/4s of the way and have to wiggle a little to get all the way up.