As usual, make sure you Prepare, Observe and Move - in that order. Use the handbrake to prevent the car from rolling. Take effective all-round observations, including both blindspots. Before beginning to move, you should be looking over your left shoulder, through the rear window.
Steering Left
Going backwards around a corner is similar to going forwards. The biggest difficulty going backwards is knowing how much to steer as it is harder to see the kerb. Plan your point of turn - when the back if the car is level with the start on the corner.
Some people prefer to watch the kerb over their left shoulder, through the side windows of the car. Others find it easier to use the left mirror to follow the kerb. Whichever method you use, you must remember to keep checking all around and to look out the back window regularly for other road users. Keep very slow.