This trick should be performed with the same type of smooth, scooping motion mentioned in the first step of the simple cascade instructions, except you aren't passing to your opposite hand. Be sure you're tossing the balls across your body, not towards your body, as it is easier to judge the catch this way, and it looks more impressive to an audience.
Practice this until you can get at least 5 throws and catches with each hand without a drop before moving on.