Wet it down for a better cut. Mist the top of your hair with warm water from a spray bottle. It can't get too wet, so douse it good.
Comb your hair straight back from your forehead. Part off the hair on top of your head and pin it up. Pin whatever's left hanging up, too, so it doesn't get in your way while you're cutting. Just don't pin it too close to the part line. Keep it low so it's out of the way.
Remove the clip from the top of your hair. Carve out a long vertical section, running straight down the center of your head from your front hairline to the part line in your crown. Comb the section straight up and snip off a bit of hair -- no more than 1 inch. Let the hair fall.
Separate out a thin, 1/2-inch horizontal section right next to the part line in your crown. Comb the sections straight up. See the shorter hair in the middle where you made the first cut? Cut the section even with the shorter hair.
Work your way toward the bangs, taking small, horizontal sections and cutting them even with the first cut. Keep going until there's no more hair to trim.
You've cut it! Now, it's time to see if you did it right. Pull the hair on top of your head straight up, using different types of partings -- both horizontal and vertical. Check for long hair. If you see any, snip it. If not, you've done a great job.
Look at your hands and fingers before each snip. Hold the hair out from the head at the same angle for every cut and keep those fingers even and parallel to the floor to avoid creating huge gashes and uneven places in your cut. Some things can be fixed easier than others. An inch of hair that's MIA is not so easy to fix.
Just want a trim? Don't take off more than an inch of hair. Take too much and you can alter the look of your cut dramatically. You don't want to change your look with this trim technique, you just want to refresh it.