I have cut/trimmed/butchered my bangs A. LOT. I've watched about a million youtube videos to get the perfect bang trim, and I finally found one that "makes the cut." You can find the video here.
The one thing in this video that made the difference for me was the angle she held her hair when trimming her bangs. Everything else I had heard before, but this was a game changer!
Easy does it! You want to go slowly and go over your bangs multiple times when you're making cuts so that it will lay nicely and blend. If you didn't take enough off go back and do it again. If you take too much off....I hope you know how the braid those bangs back (and you have enough left to do it)!