Swim butterfly using your legs only with your arms extended in front of you and one hand on top of the other.
To make this drill easier you can rest your hands on a board. Simply raise your head when you need to breathe.
Remember to make sure your head and chest do not move too much.
Try to keep your feet together underwater, so they are not pulled out of position by the movement of your knees and hips: tightening your glutes will help!