Bay parking feels like guesswork until you have reference points: fixed places on the car that line up with the bay lines.
Reversing into a bay, drive slowly past it until the line two bays along sits under your door mirror. Check all round, then steer hard toward the bay while creeping back. Straighten the wheel when the car is parallel with the lines in both mirrors.
Every car is a little different, so your instructor will show you the points in ours. Practice in a quiet car park on a Sunday morning before trying it at the supermarket.