Behind the back catches are very impressive, because you will be making the catch completely blind. But it is not as difficult as you might think.
Start with one ball. Throw it straight up, high above your head & backwards so that it will land behind you & on the opposite side of the throw. After you have completed the swing for the throw your hand should be in front of your face & you should be looking at your palm. As you release the ball watch it as it rises, peaks & falls. You'll need to look straight up because the ball will peak directly above your head. With a lot of practice & patience you will learn to judge where the ball will land just by watching the path of the ball. Simply reach back with your other hand to where the ball will land & make the catch.
With your catching hand spread your fingers out as wide as possible to create a larger catching area for the ball to fall into. Also point your fingers up slightly so that your hand makes a 'dead spider', this will reduce the chance of the ball bouncing away when it hits your hand because your fingers will act as a cage to keep it in place.
If you have to lean forward to make the catch then you are not throwing backwards far enough. Look straight up & visualise where you want to throw the ball to, lean back slightly, this will also force you to throw backwards. Don't throw out to the side too far either, the ball must disappear completely from view behind your back, if it doesn't it is merely an 'out to the side' catch. The ideal place for the ball to land is to either side of the small of your back.
Practise throwing & catching with both hands. It will take time to get this trick solid but in time you will be able to catch behind you as easily as you can in front. Practise, practise, practise...
When you can catch a ball behind your back every time, pick up the other two balls & juggle a Cascade. Pick a ball & throw it over your head exactly as you have just been practising, wait for the ball to come down before throwing the next ball, a little higher than normal to give you a bit more time & then reach back to make the catch.
To catch every ball behind the back stand with your feet shoulder width apart, lean back & look straight up. Throw each ball just as you have been practising so that they pass just in front of your face, peak above your head & fall behind you to be caught at the base of your spine. Throwing to the same height is very important, a nice even throwing rhythm is much more manageable. Do not expect to learn this trick in a day though.
Try combining behind the back catches with Behind the back & Over the shoulder throws. Also have a go at the Jon Relf Cross.