Don protective eye wear and gloves to protect yourself from flying metal and sharp edges.
Loosen the bolt that secures the blades with a screwdriver. Open the blades as wide as possible.
Place the loppers in the brace or clamp. While a clamp or brace isn't absolutely necessary, it frees your hands to work and secures the tool, preventing slips and potential injury.
Rub the blades with steel wool to remove rust, dirt and vegetative debris. Cleaning your tools before sharpening them protects your files and improves the final results, according to the University of Florida.
Run the file along the outside edges of the blade in one long, smooth motion. Start at the tip of the blade and work toward the back of the loppers. Avoid short, jerky motions and don't file the inside of the blades, which should remain flat, according to Clemson University. Hold the file at an angle sufficient to duplicate the factory-produced edge, typically 30 degrees. Adjust if necessary. As you work, you'll notice a clean edge of steel appearing. Stop filing when the entire length of the blade has a clean edge of steel.
Rub sandpaper over the edges to remove any burs in the metal left from filing. Run your hand over the blades to verify that they are smooth. Tighten the bolt that secures the blades.
Rub vegetable oil onto the blades with a clean rag to protect them from rust and to lubricate them. Store loppers in a cool, dry place, such as a shed or garage.