Wedge and String Method
Step 1. Position 4 wedges of equal length and width at the four corners of where you want the hedge to be. This may be inside the hedge which can be difficult but still doable.
Step 2. Get a length of string and tie it around the wedges at the height you want your hedge to be. Make sure the string is tied tight.
Step 3. Any part of the hedge that falls outside of the string you simply. This will leave you with a hedge exactly the shape you want and all you have to do is follow the lines.