Hedges can take several years of regular pruning and shaping to reach the height needed for a privacy hedge, but they require regular maintenance pruning even after reaching the desired height. Maintenance pruning encourages the growth of new foliage to keep the hedge looking healthy and full. The most challenging part of pruning tall hedges is pruning all sides symmetrically and accessing the top of the hedge -- some hedges can easily grow several feet over your head. This might require a ladder, which should always be used with a spotter to brace the ladder.
Place a step ladder, also called an A-frame ladder, next to the hedge on level ground. Place pieces of scrap plywood under the feet in soft soil to prevent the ladder from shifting. Enlist a helper to hold the ladder steady while you work on the hedge.
Climb the ladder and measure across the width of the hedge. Tie landscaping ribbons to the top branches at each end of the hedge to serve as guides for trimming the hedge. A good hedge width is generally 2 to 3 feet wide. If you want the top of the hedge to measure 3 feet wide, but it measure 3 1/2 feet wide, set the ribbons 3 inches in from the branch tips on each side of the hedge.
Tie a piece of string to one of the top branches with landscaping ribbon ties. Run the string down to the opposite end of the hedge, pulling it tight and releasing it from branch snags as you go. Tie the string to the branch with the landscaping ribbon at the opposite end. The ribbon might seem unnecessary, but the bright color makes it easy to see the cutting depth. You'll have to reposition the ladder several times to get the string straight. It helps to have a second person on a second ladder at the end of the hedge. Tie a string line on the opposite side of the hedge.
Tie a landscaping ribbon to the bottom branches at each end of the hedge. You don't need to use one ribbon every few feet like at the top. Set the ribbons about 1 foot farther out than the ribbons at the top of the hedge. The hedge should taper to a wider base -- make the base at least 1 foot wider than the top so sunlight can reach the lower branches.
Tie a string line between the marked branches along the bottom of the hedge. Repeat with the opposite side of the hedge.
Tie strings connecting the top and bottom string lines, using one string every 3 to 4 feet. Do not tie the strings directly to the horizontal strings -- tie the diagonal strings to the branches. Use these diagonal strings as a guide for creating an even taper along the sides of the hedge.
Climb up the ladder and trim the hedge along the top string line, working from one end of the hedge to the opposite end and repositioning the ladder as needed for safety. Cut back no farther than the string line. Repeat with the opposite side of the hedge. Use hand-operated hedge shears and bypass pruners, rather than gas or electric-powered shears that could cause serious injury if the ladder were to fall.
Trim the height back as you trim the sides to save yourself having to move the ladder. Trim the top to a dome or point, particularly if you live in an area that receives snowfall, so snow doesn't accumulate on top of the hedge.
Trim along the bottom string line guides on both sides of the hedge. You can use gas or electric hedge shears for trimming the bottom or anywhere that you can safely reach while on the ground.
Trim along each diagonal string line that connects the top and bottom strings. Work from the bottom toward the top of the hedge. Trim as high as you can reach on the ground along one string line before moving to the next diagonal string. Trim the bottom portion of the hedge before using the ladder to access the top of the hedge.
Trim back the rest of the hedge so the entire hedge is uniform with the length along the diagonal string lines. Trim the bottom first before using a ladder to access the top of the hedge. You'll have to eyeball some of your efforts to trim the hedge evenly, but cutting along the diagonal strings first provides a guide for trimming a uniform hedge.