Starting Off
The first thing you need to do is gain confidence in just moving backwards. You can gain this by practicing rolling backwards down a gentle slope, or by pushing off from a wall or other object. Make sure the skating surface is free from debris and just get used to the feeling of moving backwards. You may find that scissoring your skates slightly will help your balance.

Try and increase your speed and practice flipping from skating forwards to rolling backwards. The smoother the skating area, the better (backwards skating clicked for me on lino).

Maintaining Speed
While keeping one skate rolling in a straight line, try "sculling" with the other, pushing yourself backwards with the outward stroke. Slowly bring the skulling skate back and repeat. You can keep both skates in contact with the ground at all times, although you should eventually become confident enough to start lifting the sculling skate on the inward stroke. Most of you weight should be on the skate which is travelling straight.

Try this for the other foot, rolling on one and "sculling" with the other.

Increasing Speed
You should reach a point where you are happy to scull with either foot. Try messing around, switching feet or sculling with both simultaneously.

Skating Backwards
It is hard to explain what you should do next. All I can say is that with practice suddenly it should just come together. Once you get the rhythm there will be no stopping you. Good luck.