Starting off, your hair will have to be dry and already styled the way you normally wear it; this means your bangs will have to be placed the way you like them. When doing this, you do not want to pull on your hair, you want to just gently hold it, because if you pull the sections hard in between your fingers, you won't get an even cut. Adjust the angle that you place your fingers at with the angle that you're cutting, if you're cutting your bangs straight across and horizontally, then you will want to hold your fingers horizontally. If you're scared of cutting off too much incorrectly, use straight edged cuticle scissors because they are small and will give you the most control.
In order to shear the hair nicely, you will want to use the scissors vertically and cut into your bangs, not straight across because that would give you the appearance of a permanent helmet on your head. If you cut into your bangs vertically, you will thin out your bangs, giving them a softer appearance, and making them look wispy. Remember that when you're doing this less is more. Trim small portion of hair off at once, going at most a quarter of an inch at a time. I definitely recommend starting in the middle of your bangs and then working your way out until you reach above the middle of each eyebrow. Then continue on to the end of your bangs at whichever angle you like.