Use a shaving razor or beard trimmer to clean up the edges.
Use a shaving razor to trim your sideburns. Again, no blunt cuts. The length is really up to you. If you're going for an early-1970s Michael Caine look, then leave them at the bottom of the ears, perhaps lower. Otherwise, a good baseline is the depression just below your cheekbone. After cutting, put your fingers right underneath your sideburns to make sure they're even. The back is a bit more challenging. What you're looking to do is trim the "wings" that develop when most people's hair gets too long. Atkin suggests going into a barbershop instead of doing it yourself. In the spirit of DIY, we did it ourselves using a beard trimmer. Start on a high number setting so that the razor just barely touches your hair when you run it up the neckline. Continue with lower numbers as necessary, being careful to cut just enough to taper the hair at your neckline. Use a second mirror (or the iPhone camera in reverse) to check your progress.