Pruning young plants
Cut out any dead, diseased or damaged branches after flowering. If there’s a branch that’s crossing through the centre of the plant, you might want to prune that out too – that way you’ll get an open shrub without congestion and branches that rub against each other.
Pruning established plants
Philadelphus are best pruned after flowering in summer. It’s not vital every year, but it does promote development of new flowering wood.
You do this simply by cutting back some of the older growth to new shoots lower down a branch, and by thinning out from the base, removing a few of the older stems entirely (limit yourself to removing one in five each year).