Lay down with your back flat against the ground

Place your arms straight out to your side

Bring your legs up off the ground and bend your knees so that the both make a 90 degree angle

Keep your legs at a 90 degree angle the whole time

Rotate your knees down to the left till they are about 3 inches off the ground

Now rotate them back up and over to the right side

Keep rotating them back and forth for the desire amount of repetitions.