This is how to cut hair with scissors in a one-length manner. I've sectioned off nice, clean sections. Put your comb through and cut straight across the back. Just check your corners, that they don't go round. Just cut a one-length line right across the back. When you're cutting hair in a one-length manner, there's no elevation, and there's no over-direction. So you literally just comb down and cut in a one-length manner.
For the next section, simply parallel to the first allow the hair to fall again with no over-direction. Work cleanly. Same method. Comb in. Travel down toward the end. I can see my guide from the first section, and I cut straight across in a one-length manner. Make sure you get the corners to join in. That's how to cut hair one length.