When and how to best prune/shape Citation yews to fill out?
Pruning Citation Yews
Certified GKH Gardening Expert
The safest time to prune your Yew shrub is winter during dormancy.
You can also prune in late summer, following their spring, early summer growing.
Only light pruning is generally necessary, and if you are pruning more heavily, make sure you do not remove more then 1/3 of the shrub at any prune.
Here are some articles with more information.