How you prune a viburnum will depend on the variety.
Snowball Viburmum (Macrophyllum)
This shrub flowers on old wood, so do not prune until after it flowers. Then prune it as necessary to thin out old branches, open up the shrub, reduce height or develop a better shape. You can rejuvenate old plants by cutting them down to a short stump but, after doing so, the shrub may not bloom for two years after rejuvination pruning.
Healthy specimens rarely need pruning. Prune Viburnums after flowering only if pruning is necessary for shaping. Old or crowded plants may be thinned and shortened to bring flowers to eye level. After 4 to 5 years you may remove 1/3 of the oldest stems, and thereafter prune every 2 or 3 years.
Tree-forming A Viburnum
For a more formal effect, it is possible to train Viburnums to a single- or multi-trunk small tree. Choose the healthiest and most upright stem(s) when young and keep side shoots pinched until it reaches the height you want. Then allow the plant to branch from the top. You can remove lower branches on older specimens until you have reached the desired height and shape. Pruning on older specimens should be done during late Winter, before new growth has emerged.