Good advice above but also don't try and do it quickly...you need to keep the car moving very, very slowly. Try moving your head (lean forward briefly) to get the best view in the mirrors but don't stare in any one place more than a few seconds - you need to keep your head and eyes moving all the time to check for safety but also to judge the position of the car.
Use
all the reference points you can not just the nearside mirror - you also need to turn your head and look backwards through the rear side window
and all the way around to look through the rear windscreen. Look to see it's still clear but also where the car is in relation to the kerb. You must also look to the right for safety during the manoeuvre but also to help you judge where the car is.
Looking forward helps as well - it's another view to help judge where you are.
Once you're 3/4 round the corner you need to take off
lots of steering and from then, just small adjustments should be necessary (usually no more than 1/4 of a turn of steering) so be patient and make the car continue backwards very slowly while it lines up with the kerb in the new road.
Keep reversing (did I say very slowly?) in the new road until you are happy with the position and using ever smaller adjustments in the steering. Remember to stop with the car parallel to the kerb (looking forward again often helps with this) and when you come to a stop, the steering wheel should be straight.
Handbrake and neutral and grin at the examiner!
When asked to drive ahead, remember the normal drill you
must check mirrors and blind-spot check before pulling forward - many a learner has failed after doing a good reverse and then not checking to the right before setting off.
Hope that helps.