First, tool up!
You need a hair clipper with lots of length settings (ideally on a lock-in wheel to avoid fiddling around with separate length combs). Next, find yourself a hand-held mirror: look into it while standing with your back to a fixed mirror to check progress at the rear.
Okay: off we go..
Start off by setting the clipper to the longest length, and make smooth strokes upwards from the back of your neck. Once you’re used to the feeling, switch to your chosen length and make several passes over the same area to ensure you cut all the hair. If you’re losing your locks, a close cut will give you back your street cred. Otherwise, you can taper your style to a buzz cut by using a shorter length at the back, ending each stroke upwards into the air, before using a longer length on the front and top. Finish off by clipping your sideburns short and trimming your neck hairs (the Philips do-it-yourself hair clipper has a 0.5mm setting for this, plus a handy head that swivels 180ᵒ to let you do any angle).
Book regular appointments with your bathroom mirror to maintain your good looks.