Hi Aquagility,
If it's in the first yeay of planting, you can prune back any stagly lateral growth between now to march but not if it's frosty.
Next year, between june and late august, trim the laterals to the shape you want, keeping the leading shoots tied in to canes.
In subsequent years, june and late august, again prune back the laterals to the required shape and stop the leaders at the required height. Good Luck. Tbird+
as a conifer buff the answer to your q re yew tree pruning is:
can be pruned anytime but best early spring as new growth will cover unsightly cuts.
its worth knowing that apart from thujas the yew is the only plant that will regenerate if cut back into old wood.
this is why so many have been planted in cemetaries.
also to note for many reasons ,the berries are very piosonous so beware.