I used to work at a car rentals, and i was an utter noob at parallell parking until my workmates taught me this trick:
Here’s the setup: [car A] [pocket] [car B]
1. Drive up alongside car A, stop so you are exactly aligned with car A (the side mirrors are at the same level).
2. Stop. Get in reverse. Turn your wheel so it’s maxed out to back into the pocket.
3. Turn your head or check your rear view to make sure granny, mom with baby or your cat hasn’t stumbled into your pocket.
4. Start backing up. Look in your left side view mirror.
Here comes the trick: When you see both headlights of car B in your left side mirror, you have the correct angle.
5. As soon as you see both headlights of car B in the left side mirror, stop and straighten out the wheel. From here on you can look in your rear view mirror or turn your head and look back between the passenger seats.
6. Back up into the pocket, and straighten out as needed, the rest is easy as you will have the right angle into the pocket.
After learning this trick, i’ve never had any problems parallell parking.
Also i prefer looking back between the seats over using the rear view, as judging distances in the mirror can be tricky.