Double-Unders are one of just many skills in the CrossFit arsenal.  They require timing, coordination, and power.  Some athletes pick up the skill quite easily and some struggle with achieving them.  I find that the best way to learn the skill is to practice them with counts of four.  Try the sequence of single, single, single, double-under and repeat.  Once that sequence starts getting into rhythm, start decreasing the amount of singles between the double-under attempts.  Soon enough, you will be able to string together double-unders.  Be fast with your wrists, light on your feet, and relaxed through your shoulders.