I cut my own – been doing it for years.
It took several times before I got it right.
About an inch on top, about 1/2 on sides and edge with 1/4.
It helped me when I have cut other people’s hair.
I got a better understanding of what I do to myself.
For the back of me neck, I wrap a belt to create a line to “shave” under. That’s easier than using mirrors.