Before you begin, remove any rust from the blades with medium grit sandpaper, taking care to avoid the cutting edges as this could blunt them.
Clean the blades with warm soapy water, making sure you remove any stubborn bits of dirt or hardened grass sap.
It’s best to use a large single cut mill file to sharpen edging shears. A single cut file has rows of parallel teeth along its length. Around 250mm (10″ approx) is a good size.
A mill file is a flat, multi-purpose file with abrasive teeth that is most commonly used for sharpening blades.