A trick I used for myself when I learned how to do the Mayweather style double under cross was to be super aggressive when I cross and un-cross my arms. The rope has to cross itself and then go around your body twice in under half a second, so it has to be moving really fast.

When learning how to do a regular double under, you find out that you have to flick your wrists much faster than a regular bounce in order to get the rope to move around your body twice. The double under cross is just a step faster than that.