Yew Hedges are among the most forgiving hedges there are and will excuse most mistakes.You do need to clip or prune yew hedge plants to make them bushy - shortening side branches causes them to produce the twiggy growths that help make a yew hedge the densest and most elegant evergreen hedge there is.
On planting (October-March) cut back any side growths that straggle so you leave the line of yew hedge plants looking reasonably straight sided. In a perfect world you will leave the branches at the bottom of the plant just a little longer than the ones near the top so your hedge has slightly sloping sides. The only "DO NOT" for the time being is please, please do not cut the top of the plant off.The growing point of a yew hedge plant is vigorous and cangrow upto 40 cms in a season.This helps your yew hedge increase in height rapidly. Once cut off the plant never regains the same vigour again - if it then grows 10 cms a year you are lucky.
Joyously, you can prune yew hedge plants whenever you feel like it except in freezing weather. In the early years, the more often you clip the side branches, the bushier your yew hedge will become.We would recommend gently clipping in April, July and October for the first couple of years at least (always leaving the growing point alone, of course). Remember to keep forming the hedge to a tapered shape to allow light to get to the lower branches.
When your yew hedge has reached the height you want (best practice would be to let it grow about 10cms higher than you think you want) you can cut the growing points off. Reduce the height little by little as if you take too much off, you may be waiting a year or two for it to grow back.
Thereafter, just clip when your yew looks like it needs a trim. No hedge is easier to keep in shape or forgives mistakes more readily.
Should you wish to drastically alter the shape or height of your yew hedge - for example cutting an archway through it, or significantly reducing the height, carry out the surgery in mid-winter when the plants are semi-dormant. If you have an overgrown yew hedge and you need to reduce its width, cut one side back very hard indeed one winter, right back to the tree trunk in the middle of the hedge and reduce the other side the following year. Remember that yew will put out new growth even from centuries old wood - even the most awful mistakes you might make will self-heal and disappear...