Most car parks have rows of parking bays neatly amrked out with white lines. Reversing into such bays is noramally the best option ai it is easier to manoeuvre in reverse, and it is also safer to drive out forwards than backwards.
The only advice I can give about how to do bay parking is to keep practicing it until you can do it easily. Different instructors use different methods varying from having markers in or on the car or using existing body details to help with positioning, to having no markers at all and getting the position right by driving correctly. Follow whatever your ADI normally does, because he finds it successful and is used to it.
In any of these manoeuvres there is little point in asking to do it again. If it really was bad, it is sufficient to cause failure on its own. If it wasn’t, and there is nothing else, then it won’t be used anyway. There’s always the chance that the second one will be worse, and the examiner can’t ignore that!