what is best time of year to do this
The best time to prune yews, and other evergreens, is late winter or early spring, before the new growth begins. Pruning during this dormant season, allows the trees time to recover before active growth begins.
To maintain size, prune in late winter or early spring, before new growth begins. Prune again in mid-June. Annual shearing should be done after new growth has expanded. Follow-up shearing should continue throughout the growing season. To avoid stimulating new growth late in the season (thus preventing winter injury), do not shear after August.
Below is a link to the answer to a previous question on Pruning Yews, which contains details of how to prune.