There are two things you need to do to master the behind the back pass.
The first thing is to strengthen your fingertips. You can do this through dribbling and passing drills.
The other thing you need to do is continuously practice your behind the back pass in training and pick-up games. There are limited drills when it comes to behind the back passing, so it’s something you need to practice with friends and teammates. Just like shooting and ball handling, it is not developed overnight.