Some lawn mowers such as the Craftsman 37705 have a washout feature. It is essentially a garden hose fitting on top of the circular blade cover. This allows you to pump water into the blade assembly while running the engine. That action is designed to flush dirt and grass debris right out.
Get out there and mow Now that you have done all that your mower should be ready to tackle the season in stride. It should run smoother and more safely too. Even better you have just saved the $200 to $300 fee it typically costs at least in my neck of the woods to have your lawn mower serviced professionally. So go on and get out there. That high grass needs cutting.
Next you will change the oil. To lower oil viscosity and get it moving freely run the engine for a few minutes. Now shut the engine off and roll the mower onto a piece of cardboard or drop cloth. Close the fuel line (if your mower has one) and unhook the spark plug as before. Find the oil filler tube and remove its cap. Often the cap also functions as a dipstick.