Glass files are amazing. They give your free edge a really smooth finish, and they avoid any kind of hangnails at all. I recommend purchasing one the first time for about maybe, like, $2 or $3, because if you drop it, they're going to break.
You take the finger, as always, and you file in the same direction. Well, actually, I think you actually have the option to go back and forth with a glass file, because it isn't going to create that friction that you have to worry about. And the fragments.
As you hold it, you can literally feel how smooth the nail is the first time you use it. It feels really good. The only thing is that they wear out kind of quickly. That's why I wouldn't recommend investing in . . . spending too much money on them, because you're going to replace them. Although they say that they rejuvenate themselves, I disagree.
But they're also sanitizable, so you don't have to throw them away. You can wash them with warm water -- with soap and water -- and you'll see that all of the dead skin will come off. But the smoother they get, the harder it is to file a natural nail, especially if the nails are strong. But believe me, when you're using it, you'll feel the difference in how smooth it feels when you're done.
If you're keeping them in your bag, always get a little sleeve. This way, they don't break in your bag. And God forbid they do, they break in the sleeve and not in half in your bag. Remember they're glass. Treat them special.