I cut my own also, along with my SO, and in a while, our daughter’s.
I’ve used a variety of ways to look at the back of my head. The best I’ve used was mirrors front and back. My hair is about 3" long, no buzz.
I start off with a wet head, scarier that way, but much easier. I cut my bangs, by pulling them straight down, then the top, I begin pulling up sections of hair and cutting them pretty much at the same length, don’t grab to much at a time. I tend to get a little shorter as I reach the back.
On the sides, I comb it straight down, over my ears. Pick a point at the front that will go behind my ears. I open the scissors, and lay them so the one point is behind my ear, then whilst holding my ear down, I snip it off. Keep a hold of the ear, and wrap behind.
Now for the back line, This requires a lot of patience. Starting at one ear, (for me it’s the right ear, ymmv) cut down at about 45 degrees, begin to roll to a flat to the center of the neckline. Continue flat for as long as possible. Now go to the other ear, and do the same 45 degree, but roll the corner to the flat. Final check, angles, line, roll and trim a minute amount to even things out.
Up the head to the back. Cut pretty similar to the top. Pull it out, guess how long it is, and cut. Again small sections at a time so mistakes aren’t noticeable.
Almost finished now, but it gets interesting, the NECK.
I sometimes do my neck with the same razor I do my face with. It’s not to hard, and if not for my mole, I’d never have cut myself. When shaving one’s neck, hold your newly cut hair up against your scalp, so you don’t ruin your nice line. Then just wet neck, as I will have dried out by now, apply favorite cream or gel, and cut from the shoulder up. Shave up as far as you can go, without cutting the hair you are holding up. I also use a beard trimmer for this task. Less scary, but not as clean imho.
Now comb it, and trim up the loose ends. Pull loose ends from around the top back of ears, then cut. Check bangs, trim as necessary.
Remember when watching yourself in a double mirror, practice cuts prior to making the real cut. If it looks funny in the mirror, don’t bring them blades together. Watch your ears!
Oh yea, I use short sewing scissors, and a twin blade razor.
Wow that got long!
Dan
PS, no one ever suspects I cut my own hair anymore. When they find out, the sometimes still don’t believe it.