Summarize this article:

You can find an emery board online or at a local drug store. It's best to buy an emery board that is not made of metal, as metal might cause damage. Some dentists do not recommend emery boards, because enamel is hard and may need something sharper, but it may be an OK option if only a small amount of filing is needed. Stand in front of a mirror so you have a good view of the tooth you wish to file down as well as the emery board. As with using a nail file, you want to be sure that the tooth in question is pain-free. If you are experiencing pain in the tooth, it is not a good idea to continue with filing. You want to file down the tooth so that it is no longer cutting your mouth or tongue, but once that point has been reached it is best to stop. Be conservative with your filing, as you do not want to damage the enamel. If the tooth is causing pain, it is possible you have damaged the enamel. This can lead to dental problems in the future, so it is a good idea to go see a dentist.

Summary:
Buy a non-metal emery board. Place the emery board flat against your tooth. File down the tooth with a few back-and-forth strokes. Pay attention to any pain in the tooth.