At first it is important to breathe naturally. Too much focus on breathing can bring the mind away from the body movements. It is important not to think too much (even about breathing) or one will loose the form.

When hands go up breathe in, and when hands go down breath out. When closing the body inhale, and when opening the body exhale. When moving up or out breath in. Breathing is a bit like being a bellows.

When breathing the abdomen moves in and out with the breath so that breathing is vertical.; the breath moves vertically. If only the chest goes up and down the breath is horizontal and is not deep enough.