Obviously this comes with a warning, which is: not many people will advise you to cut your own bangs. I've been cutting my own hair and my own bangs for many years, so while I'm no licensed hairstylist, I feel super comfortable wielding a pair of scissors near my hair. I also have curly hair, which is rather forgiving in the haircut department. My methodology of hair cutting has always been one that is for big, curly hair, so it may or may not work for you if you have a different hair texture. That being said, I feel like my way of cutting bangs could definitely work for hair that isn't my same texture, but, you know, use your best judgment.
Here's how I cut my bangs:
1. Separate the section of hair you want to cut into bangs. I typically do a triangle shaped section with the point near my crown. This will determine how thick your bangs are, so the bigger chunk of hair you grab, the thicker your bangs will be. You also may want to take into consideration how thick your hair is, since the hair you take away to be bangs will no longer contribute to your hair's overall thickness. Once you have your bangs section separated, tie back the rest of your hair to keep it out of the way.
2. Flat iron your sectioned hair. This may not be 100% necessary for some of you, but since I have curly hair, it is. Most hair stylists will cut hair (and bangs) wet, but since my hair is curly and poofy, I prefer to cut my hair with it's poof and texture intact so I know how things will look. When my hair is wet, it's way more limp and longer than it is when it's dry, so if I cut it limp and long, it gets shorter and poofier when it's dry and the cut doesn't look anything like it did when it was wet. I never cut my hair wet, so this is just a personal preference.
3. Rough in the shape you want. I typically do bangs that are on the long side and are tapered longer on either side. Since I have curly hair I sometimes will wear my bangs with their natural curl and they appear much shorter when they're all crimped up in their natural curl. Keeping my bangs as long as I can handle helps me style them curly (without them looking like I have a tiny 'fro on my forehead). When roughing in the shape, leave the hair longer than you want it to be when you're done. You just want to get the general shape, then we can go back and perfect the shape. Remember, you can always cut shorter, but you can't cut longer. Always err on the long side, if you live in your bangs for a few days and decide you need them to be shorter, you can always go in and give them a little trim.
4. Make sure that both sides are the same length and hit your face at the same spot. Then, from the middle, you can use your fingers to pull the hair down and match lengths as you trim down towards the longer sides (if you're doing a tapered bang like mine. If not, just use your fingers to pull the hair down and match the lengths of each piece you cut to the piece that you previously cut).
5. Instead of cutting perpendicular to the hair, cut at a slight upward angle. Unless you're looking for a super blunt looking bang, a la Sia. It'll be a little slower going because you're cutting fewer hairs per snip, but you'll get a much better, more natural looking cut than if you just go straight at it.
6. If you need to, you can go back in with the flat iron to give your bangs the shape/curve you want them to have to help you finalize your cut as you snip in your final shape.
7. Again, I'll advise you to cut them slightly longer than you might want, and then live in them for a day or two. Unlike going to a salon, you don't have to have your bangs absolutely perfect when you're done. Since you're the one cutting them, you can go back in at any time to edit them. I cut mine a little long and I've been living in them for about a week and I feel like I probably need to cut a few millimeters off so they're perfectly out of my eyes.
Fringe upkeep is just a matter of repeating that final trim process when your bangs get too long! And make sure you have a decent pair of scissors. Of course, this is coming from a girl who used to cut her own hair with paper crafting scissors back in college. You can grab a nice pair of scissors from a beauty supply store, or even sometimes the beauty section of a store like Walgreens or Target.
Be patient with both the process of cutting bangs, and with getting to know your new cut! Bangs can be a big change and it can take a while to get use to how to style them and wear them. Have fun!