Well you don't, really, its not necessary to prune these (assuming you mean Pieris 'Forest Flame'). You can reduce its size a bit now if you want to though.

Welcome to Goy,John...I've trimmed both mine (Forest Flame)over the years,but mainly just to reshape them,or remove unwanted growth.As Bamboo suggests,it's fine to do it about this time of year,they will be fine..