Mock orange is a family of flowering shrubs so named for their fragrant white flowers that resemble orange tree blossoms and appear on the shrub in profusion from late spring into the summer. Mock orange is a deciduous and semi-woody shrub and, according to Purdue University, it should be trimmed for height and spread in the later summer or early fall after all the flowers have died back.
Keep a mental tally of the amount of plant material you are removing and keep the total amount of plant tissue you remove to under a third of the overall shrub volume. This will lessen stress on the shrub and prevent a shock response from occurring. If you wish to reduce the shrub further in size you can remove another third the following year.
Trim the terminal branch tips, the tapered ends of the branches where new growth emerges. Work branch by branch, removing up to one half of the branch length, as desired, but no more. Work all the way around the shrub, following its natural form until you have reduced the overall height and sprawl to better suit the setting or your liking. Make all cuts on the bias and roughly 1/4 inch above a leaf node or healthy bud.
Cut out a handful of the oldest or most defoliated stems down to the crown of the shrub, just above the soil line, to encourage new bushy growth with a full set of green foliage.
Prune away any dead, cracked, diseased, abrading or entangled branches down to the parent branch or all the way down to the crown of the shrub as desired.
Pull all of the cuttings from the shrub canopy gently and rake up any cuttings or foliage that has fallen onto the soil surface below. Removing decaying plant tissues will deprive insects and opportunistic diseases of a breeding ground. Discard or shred and compost any cuttings.