If you want your hair to look great, you need to know how to cut bangs so they look appropriate for your hairstyle. Whatever you do, don't cut them too short. You can always cut a little more, but you can't put hair back once it's gone!
The trick to getting good bangs is to use facial features to measure where you cut them and use them as a guide. The large (bumpy) bone on the bridge of your nose is where your hairdresser typically measure's them from, especially if you want your bangs to fall just above your eyebrows. If you want longer bangs, cut bangs using the apple of your cheek bones as a measuring point. Optionally, if you want longer bangs, use the jawline as your point of reference.
Bangs can be cut either wet or dry. Because wet hair shrinks after it gets dry, always measure longer than what you really want. That's why using facial features works so well as a measuring point. Hairdressers (most of the time) cut hair when it's wet. You can also cut it when it's dry if you like to see what you're gonna get immediately.
Always cut curly hair longer than what you think! You can always go back and cut more later if they're too long!