How to Do Double Unders
If you want to do a double under, you'll first need to master the single under. To do a single under, stand with your feet hip-distance apart and your elbows close to your sides, then jump with straight legs so you're about 1 to 2 inches off the ground. Once you can consistently do 100 single unders in a row, advance to the double under by practicing a higher jump that will give you more airtime. While you jump, focus only on flicking your wrist as quickly as possible, so you can eventually turn the rope twice in one jump. You should also make sure to time your double under so you are jumping as soon as you flick your wrists downward.
Choose a rope that's your height plus  3 feet (0.91 m) feet. Length and weight are important factors in jumping rope successfully. When standing on the center of the rope, the handles should come up just below your armpits. It doesn't matter if you use a lighter or heavier weight. But consistency is key - sticking to the same weight during your practice will help your body memorize the movements more easily. Thinner, lighter ropes will move faster. However, some people like the control they get from the heaviness of a thicker cable. It all depends on your personal preference. More advanced jumpers might try speed ropes to help further develop their technique. Stand with feet hip distance apart with your body tall and relaxed and elbows close to your sides. Swinging your arms away from your body not only tires you out faster, but it actually shortens the arc of the rope, making it more difficult to rotate it quickly enough. Focus your eyes on a point straight in front of you, not looking down or up. Your hands should be slightly in front of your hips and below your elbows for quicker rotations of the rope. Jump with straight legs, keeping them directly beneath you. Straight legs - as opposed to bending your knees - will also allow you to bound up quickly and more efficiently. Push off the ground with your calf muscles and land softly on the balls of your feet. It shouldn't sound like you're stomping when you land. Avoid the common “dolphin kick” mistake. Also known as piking, this is when you kick your feet out in front of you when you jump up, throwing off your timing along with your next jump. You don't have to jump super high - 1 inch (2.5 cm) to 2 inches (5.1 cm) inches off the ground is enough. Practice a minimum of 3 to 5 times per week. It can be frustrating when a new skill is difficult to learn. But practice does make perfect, especially in the case of double unders. Set aside time at least 3 days a week to work solely on your jumping or practice for 5 to 10 minutes before your daily workout. Once you can consistently do 100 single unders in a row without failure, you're ready to move on to the double under. Learn the power jump to get more height and air time. The higher you jump, the more time there is for the rope to pass underneath of you. Use your legs to propel you up for a jump that's about twice as high and twice as long as your single under jumps. Fight the temptation to flail or kick up your legs - instead, maintain the same posture you used for the single under. To build up power and stamina, set the rope aside and do box jumps or tuck jumps. They use similar motions and strength and are an effective way to train your body. Focus on only moving your wrists. Contrary to what most people think, the key to turning the rope twice in one jump isn't how fast you move your arms: it's how fast you rotate your wrists. A quick flick of the wrist is all it takes. Practice your wrist rotation with isolated drills. Hold 2 pieces of pipe in your hand and imagine you're painting the inside of a bucket with them, moving only your wrists in small tight circles. Talk about a fast flick: The world record for double unders is held by Shane Winsor who completed 164 in one minute. Jump as soon as you flick your wrists downward. The number one thing that messes people up on double unders is the timing: They either jump too early or too late. As you start to rotate your wrists toward the ground, you should already be preparing to jump back up. To help you stick to a rhythm, listen to a metronome app on your phone or pick a song with a steady quick beat while you jump. Increase the number of double unders. Once you've mastered one, begin slowly integrating more double unders into your jumping. Start with two in a row, then three, etc. Time yourself while doing a set number of double unders. See how long it takes you to do 50, 100, or even 1,000 double unders. Then, try to beat that time during your next workout. Just make sure you don't sacrifice form for speed. This method of "for time" workouts is good for increasing endurance. Swap traditional cardio for interval training 2 to 3 days per week. Set a timer and alternate one minute of double unders with one minute of single unders for a high-intensity interval training (HIIT) workout. Benefits of HIIT include a healthier heart and increased fat burn up to 24 hours following your workout. You can also mix in other plyometric moves like high knees, air squats or jumping lunges. Try a CrossFit "Workout of the Day" to build strength and conditioning. Commonly known as WODs, these intense routines are posted daily and often include double unders mixed in with exercises like sit-ups, rowing, and squats. Do a WOD daily for three days in a row, then take a rest day. Build your own WOD by combining double unders (cardio) with clean and jerks or dumbbell snatches (weight training) for a full-body workout. 
Choose a rope that's your height plus  3 feet (0.91 m) feet. Stand with feet hip distance apart with your body tall and relaxed and elbows close to your sides. Jump with straight legs, keeping them directly beneath you. Practice a minimum of 3 to 5 times per week. Learn the power jump to get more height and air time. Focus on only moving your wrists. Jump as soon as you flick your wrists downward. Increase the number of double unders. Time yourself while doing a set number of double unders. Swap traditional cardio for interval training 2 to 3 days per week. Try a CrossFit "Workout of the Day" to build strength and conditioning. 