Assess the amount of new growth. It's easy to do: the current season's growth has leaves attached, while the old wood below is leafless. The top and upper sides of a hedge are exposed to lots of light, so they grow more vigorously. They need to be pruned harder than the lower parts of the hedge.
Prune back each shoot to two or three leaves from its base, using sharp secateurs. Cut at an angle next to a bud, with the lowest point of the cut positioned opposite and even with the bud, and the highest point just above it.
Work your way across the full width of the hedge, holding your pruning arm at right angles to your body so that you maintain a level top. Stand back regularly to check that the height is correct and even. If you find this difficult to do by eye, use a string line as a guide.
Remove prunings from the hedge as you go, so they don't get in the way. Once the top of the hedge is complete, it's easy to cut the sides. Start trimming at the top and work down, aiming to taper the sides to leave the base a little wider than the top. You may find a bamboo cane useful as a straight line to cut to.
Check the hedge and prune back any stray shoots that you might have missed. Rake up the leafy prunings and add them to the compost heap. Clip them into smaller pieces if necessary. They should rot down quickly to make a useful mulch to spread beneath your hedge next autumn.