[header] How to sharpen lawn mower blades [title] Disconnect the spark plug and power source. [step] A lawnmower can cause serious injury if it starts while you are working on it. Always disconnect the spark plug and the power source (outlet or battery) before disassembling your mower. 
[substeps] The spark plug can usually be disconnected by removing a prominent wire on the side or front of the lawnmower engine from its metal mounting. Once disconnected, the engine should not be able to start.