First, grab a pair of straightedge cuticle or mustache scissors. Style bangs as usual and clip back the rest of your hair. Start at the center of your bangs and work your way to the outside.

Using your fingers, gently hold a small section of dry hair - only about 10 to 12 hairs at a time - straight down against your forehead. (Don't pull too tight - you'll stretch the hair and end up cutting a lot more)
Trim and cut below your fingers to keep from cutting it shorter than you'd like.

When you are done trimming each section, comb it back into the rest of your bangs to check length (and that they are even) before moving on. When you reach the end, return to the center and trim in the opposite direction until you are done.

Article courtesy of Free Beauty Tips