You have a choice - you can cut it to the shape and size that you want it to be at this time of year, and lose some of the flowers, or you can wait until after it's flowered. It takes pruning well. Some people grow it for the foliage and prefer to prune it in spring. It's up to you! The 'how' is to cut out dead or damaged stems, and then shape the shrub, cutting just above a good bud.