Before sharpening and balancing your blade, you should always make sure that you've scraped your lawn mower blade free of grass, dirt and other debris. This will make the sharpening and balancing process much easier.
Here's a quick recap of the video:
How to Clean Mower Blades
Alright, we're going to break out that putty knife again (it's a common tool to be used for cleaning a mower). You'll want to scrape off all of the extra debris off of the blade and make sure that it's fairly clean. Try to get into the grooves of the blade and clean those out as well