Your Own Hair
Begin by shaving the sides and back of your head with some electric clippers. Michael Gilman, founder of the Grooming Lounge, suggests doing this on dry hair so you can see the results in realtime. Use the guards to maintain length and start out with a longer guard setting. Then go down a guard (or two) to get a little closer at the bottom around your sideburns, ears and neckline to create a slight taper. Remove the guard and clean up the nape and any errant hairs on your neck.
Divide and Conquer
Make like a barber and slightly wet your hair. Then make two parts, one at the outer edge of each eyebrow, combing the hair above the part into the center of your head. This will keep you from cutting the top too short. Then comb the hair at the sides forward towards your temples and trim any excess hairs sticking out from the natural hairline. Finish by snipping any rogue hairs around your sideburns or over your ears.
Tackle the Top
Comb the hair straight up and start cutting into it perpendicular to the comb. This will prevent any blunt, straight cuts and also give your hair a nice texture. It also alleviates a lot of the risk of chopping off too much in one snip. Like with the clippers, start off conservatively and take more off if needed. As a rule, you want to keep the front a touch longer than the hair in the back toward the crown of your head.