It is important to keep your lawn mower blade clean and in good repair. Doing so will not only leave you with a nicer, more evenly cut lawn, but it will also help in cutting down on the spread of diseases that might negatively impact your grass. When you cut your grass, you leave it susceptible to disease and contagion via the open wounds--and just like with people, open wounds in grass mean a greater chance for bacterial infection. Cleaning the lawn mower blade will significantly reduce this chance.
Put on heavy work gloves. You will be working around a very sharp blade, so be sure to protect your hands with sturdy work gloves.
Empty the gas tank on your lawn mower. To get to the blade for cleaning, you are going to need to turn your lawn mower up on its side. If you don't empty the gas tank first, it is likely you will end up with gas spilling out. You can either run the tank dry or use a hand pump siphon to extract the gas from the tank before starting.
Remove the spark plug wire from the spark plug. Since you will be rinsing your lawn mower down, you don't want any current running to the engine. This will also eliminate any risk of the lawn mower accidentally starting when you are working on the blade. For most lawn mowers, this is accomplished by firmly grasping the spark plug wire and pulling upward. Some lawn mower models might require the use of wrench to remove the spark plug wire.
Spray down the housing and blade with a hose. You will want to get all of that crusted-on grass and debris off the housing and blade before going any further. A hose with a relatively high pressure stream will work great to knock all off that accumulated gunk off your lawn mower. Just lay the lawn mower over on its side and spray until the great majority of the debris has been washed off.
Brush down the housing and blade. Use a stiff-bristled brush to clean the last remnants that the hose simply would not remove.
Remove the blade from the lawn mower. Use a ratchet set to remove the blade from the housing. This might be difficult and require some serious leverage, so using a long-handled ratchet wrench is recommended.
Wipe down the blade thoroughly on both sides. You might wish to sharpen the blade while you have it off the lawn mower. You can either do this yourself with a whetstone or sharpening tool, or many do-it-yourself stores will be willing to sharpen the lawn mower blade for you for a minimal fee.
Wipe the housing and blade down with a thin layer of vegetable oil before reattaching the blade. The vegetable oil will help to prevent debris from accumulating on the blade and housing as quickly in the future.
Reassemble the lawn mower. Reattach the blade, making sure to tighten it down thoroughly. Sit the lawn mower upright and reattach the spark plug wire to the spark plug. Fill the gas tank and you should be ready to go.