To do Double Unders, hold an end of the jump rope in each hand. You may then want to start with a few basic jump rope jumps before beginning Double Unders.
Then begin to spin the rope faster to spin the rope twice under your feet as you jump up. You may want to jump a bit higher as you begin to learn the Double Under.
However, you do not want to spend a ton of energy jumping extra high with each Double Under. Instead you want to work on spinning the rope faster as you stay relaxed and jump only as much as needed.
Beginners will need to start by spinning the rope faster and potentially jumping higher. They can even begin to learn the Double Under by performing a Tuck Jump.
As you do Double Unders, stay up nice and tall and really spin the rope quickly using your wrists while keeping your shoulders relaxed down and back as much as possible.