I did military cuts for men in my division when they couldn’t get an appointment with the barber. This was not boot camp cuts. They were allowed to have some hair, more than Marines. I made the back look not shaggy by going over it with a disposable razor. I didn’t apply pressure, I just pulled it lightly down the back to the level of the neck where it was supposed to end, and be shaved. It gave it a good smooth appearance, even with curly hair.
If you want advice on how you do yours, you need to describe yours, thickness, curly or not, etc., and how you want it to look.