Drive around until you find a spot that looks big enough.
Pull up even to the front car. If your cars are different lengths, line up the back of your car with the back of the front car as best you can. You don't have to be exact here.
While stopped, turn your wheel all the way to the right. ALL THE WAY. Don't move forward or back while doing this!
Turn around and look out the back of your car.
Begin backing up. Your car should start turning into the spot. Don't turn your wheel away from the all-the-way-right position!
Stop backing up when the right-front corner of the rear car is in the exact middle of your rear windshield. If you imagine a line extending backwards from your car along its centerline, you stop when the right-front corner of the rear car reaches that line.
I said STOP.
While stopped, turn your wheel back to the middle position.
Back up slowly until your car just barely clears the front car, then stop again.
Turn your wheel all the way to the left. All the way! Stay stopped while you do this.
Now keep backing in. Don't turn your wheel away from the all-the-way-left position!
Once your car is parallel, STOP and then turn your wheel to face forwards again.
If you do these steps exactly, your car will magically place itself into the correct position.