How do you learn a backhandspring by yourself without a spotter?

Answer 1

I'd really recomend you don't. It would be very risky, you could break your neck.

There are ways you could make it safer, like learning a bridge kick-over and back-walkover first, by doing those down a "cheese" (foam wedge) to start with and then on the floor. You could practice "popping" your shoulders in a handstand. You could learn to do a round-off and a handspring so you have a couple of moves where you get to your feet by pushing off your hands.

Then you could do the backhandspring from an angled trampette or two stacked spring-boards onto a couple of crash mats topped with a firm floor mat that will give enough stiffness to allow you to push away with your hands.

But if you had access to that sort of kit you'd have access to spotters too. So what you are really asking is probably how to learn a backflip with no safety aids at all, human or otherwise. And the answer is that if you like your spine intact then you don't.