trimming bangs

How long are your bangs? For the swoop look they need to be several inches or longer. I trim my wife`s hair monthly and also a couple of her friends` hair on occasion, mostly repairs from salon disasters. Why is it the paid "professionals" always seem to screw up so much?
1st use a very sharp pair of scissors.
2nd, dampen your hair.
3rd comb your hair to the desired part, then hold it up above your head above your ear opposite of the side of your part. Now check how the ends are. The side away from the part is probably much longer than the hair from where it was parted. Trim the longest hair sticking up even with the shorter hair, then let your bangs down.
How does it fall? If it is lays alright, leave it. If you want more off, just repeat trimming a 1/4 at a time.
You can always trim more, but you cannot untrim it.