You should use a file to sharpen the blades of your reel mower. Make sure the blades are clean and dry before using them. Follow the edge of the blade with your file in only one direction. Instead of grinding away the metal evenly, do not go back and forth like you would with scissors or a knife. If you have access to an angle grinder or bench grinder, you can also use them. Use these methods with caution - they're powerful and can cause serious damage if misused.