Divide your hair into a triangle section from the top center of your head to the outside of your eyebrows. For thicker bangs, start the triangle farther back on your head. For thinner bangs, start the triangle closer to the center of the front of your head.
Clip back the side sections to avoid accidentally cutting any of these hairs. Clip back sections of the hair in front, starting at the back of the triangle and working your way down. This is so you can trim your bangs section by section, which is the key to getting a straight-across cut.
Take one section of bangs between your fingers. Hold the scissors vertically, and start cutting up from the center of your hair. Work your way to the outer edge on either side. Take down another section of clipped hair, and lay it over the first. Cut the new section of hair to match the first without cutting the original section. Repeat with all the sections of bangs.