Posture!

Stand tall, friends! Because hunching over looks unhealthy. Because you will get no repped at the end of your squat if you don’t. Because jump roping while standing tall is more efficient that crouching.

If your posture is bad when you do singles, it is just going to get worse when you try doubles. Take your time. Work on standing up straight while doing slow singles. Once you’ve got that nailed down, do alternating singles, do crosses, play around and stand tall. The better the posture, the less effort required to whip the rope around.