Wet down your hair with a spray bottle or save yourself the trouble and cut your hair right after you've shampooed and conditioned.
Comb through your hair and part it in the middle. Create another horizontal part across the top of the head, running from the top of one ear to the other. Comb all the hair before the part forward. Pin up the hair in the back, leaving a 1/2-inch row of hair hanging free in the nape.
Tilt your head forward -- you want to keep it this way any time you cut nape hair. Comb your hair straight down on your neck. Do not lift it or elevate it from the neck even a smidgen. Trim off about 1/8 inch of hair. You don't really want to change the length or the shape of the cut you already have, so don't take too much.
Loosen the hair clips and bring down another 1/2 inch, horizontal parting of hair. Comb the hair straight down. This time, lift the hair up from the neck, holding it between the first two fingers of your left hand before snipping. Hold it approximately one finger width from the neck. You should be able to see the hair you previously cut through the longer hair on top. Cut the hair even with the previously cut hair. You will remove more than 1/8 inch of hair due to the elevation that you're holding the hair at, so don't freak out. You're supposed to take off more this time.
Pull down another 1/2-inch row of hair. Cut the hair at an elevation of two finger widths from the neck.
Keep pullng down 1/2-inch rows of hair, cutting them at elevations that increase by one finger width each time until you've cut all the back hair.
If the scissors aren't sharp, don't try to cut your hair with them. Dull scissors push hair instead of cutting it, resulting in an uneven cut.