Start with DRY hair (if it is wet you will cut bangs too short) pulled back, leaving out the pieces you want to cut. Include some longer side pieces to frame your face.
Separate the main part of your bangs from the side bits, in the front. This doesn't have to be perfect, but should come from your side-part and be about 2-3 inches wide.
Diagonally lay your finger across your hair where you plan to cut (you will cut at an angle) and hold against your face to imagine length. Leave your finger here and pick up scissors.
REMEMBER: Cut your hair so that it is LONGER than you may think you want! Wherever your finger is, move it down about half an inch. You can always cut again if it's too long, but you can't un-cut it!
Hold your scissors at the same angle your finger was (move your finger away) and cut across. This will leave a blunt edge that we will fix in the next step, but gives you your length.
Holding scissors straight up and down, cut into your bangs to soften the ends. Move your scissors up and down to vary the length of hairs, and don't overdo this: only cut until the ends are less blunt
Onto the side bits! Hold them next to your face to judge evenness and where you'd like them to fall. Right around jaw/chin length is usually good. Remember your measurements!
Do what you just did with the main bangs: cut at an angle, then cut vertically to soften. Be careful on the last step as you have less hair to work with, and could cut too much!
Ta-da! You're done, with a new haircut for free!