Q: What is the proper way to prune our yews?
A: Yew is one of the few conifers that will sprout vigorously from the bare limbs and trunk when cut back hard.
That said, yours don’t look like they need a hard pruning, just a re-shaping.
It looks like you have, over time, allowed the top to grow wider than the bottom. This caused the lower branches to thin out, so now the shrubs look like inverted triangles.
Wait until late February and trim the front side back by a foot. Remove six inches from the top and back side.
The result, later in spring, will be a vigorous mass of greenery that is more symmetrical.
In the future, try to keep the top slightly narrower than the bottom, to allow sunshine to hit the lower limbs.