'Columns' is aptly named as each ball goes up and down in its own column. This pattern introduces an important concept - throwing two balls at the same time. This is known as a 'synchronous' or 'sync' pattern, whereas the cascade, and other patterns with alternating left/right throws, are called 'asynchronous'.

Practice throwing 2 balls to the same height. There's really not much more to this step.

Practice juggling 2 balls in one hand. For the sake of this pattern try to keep them in separate columns, though you should practice with inside and outside throws as well.

Once you can run columns you should practice using different hands for the middle ball.

In addition to alternating hands, incorporate inside and outside throws. Columns provides a solid foundation for exploration of many different tricks.