The following is what I like to call the double shrimp squat, sometimes also called the advanced shrimp squat. I first picked up this move from Al Kavadlo.
The single shrimp squat, that is with just one hand behind you holding onto the foot is an easier version and a preliminary step to this one. It takes less flexibility to do.
Even more preliminary is the airborne lunge described here.
Some people will already have all the flexibility they need to pull off this movement. Just give it a shot and see how you do. For others, you’ll have to seriously work on your flexibility, especially in your ankle and also in your hip flexors to pull it off.
It takes some strength, but most people that have done any sort of leg training already possess the leg strength it takes to move their whole bodyweight up and done.
The shrimp squat series is just one of four different variations of one-legged squats. Doing each one of these can help with the others, though they all have unique differences.
For much more on bodyweight squat training, check out The Ultimate Guide to Bodyweight Squats and Pistols.