There are many methods of learning to unicycle backwards:
1) The controlled way: First learn to idle the unicycle by putting the pedals horizontally and controlling the pressure on each pedal so that you stay in the same place. Then, rock the pedals back and forth, so that you start to move backwards, forwards, backwards, forwards... Then finally, when you feel ready, rotate the pedals backwards a half-rotation. You should be controlled enough to get the unicycle idling again straight away. Build on this, and soon you'll be able to go backwards and forwards at will.
2) Using a wall or another person for balance. (Just like you did when learning to go forwards).
3) The "Just Do It" method: First, make sure there's nobody within a ten meter radius.... 8^)