Lift the cheek slightly and pull the short down enough to expose the cheek.

I mooned in response to a guy whistling at me. I was on the right side of the left turn lane and the guy whistled at me from the passenger seat of a car as I pulled beside them. Without hesitating, I pulled the moon and backed up at the same time. The woman driving the car and the teenage girl in the back seat were crying with laughter. The guy who whistled was beet red and slinking down in his seat.