Use scissors or shavers that are designed for hair cutting and have sharp blades. Just follow the pattern of the person's head trimming the hair along areas such as near the ears and above the eyes. There are tutorials that come with many male hair-cutting kits that provide step by steps directions.
the easiest way would be to shave it. otherwise, use a scissors to measure the length you want and cut along the scissors, so that the cut is straight, to thin it out cut a little shorter in random places throughout.