I respect hair stylists as the artists that they are, and in no way can I claim to be able to replicate salon hair. That being said, I'm big into bangs these days, and there's no way I'm paying for my daughters and I to go get our bangs cut every few weeks. So here's how I cut bangs:
First, comb the bangs out so that you know about how much you'd like to take off. I like bangs to just brush the eyebrows, so I needed to take off about an inch here, maybe an inch and a half. Second, wonder to yourself how you let your kid's bangs get so long.
If you like something more exact, I start at the right and imagine that the line of the eyebrow extends to the hairline. I cut along that angle until I meet the eyebrow, then follow across the eyebrow line, going down at the left the same way - following the angle of the eyebrow. I usually do a combo of both, and take the length off with the twirl method, then even it up a little with the "Follow the Eyebrow" method.