Lawn Mower 21-In Deck Mulching Blade

Disconnected spark plug, replaced spark plug with new one, kept it disconnected.
Un-capped gas tank, placed ziploc bag over gas tank opening, screwed gas tank cap back on.
Tilted mower on its side, air filter up.
Wedged block of wood between a bolt on the underside of the mower deck and the old mower blade such that when I loosen the mower blade bolt in a counter-clockwise direction.
The blade bolt is tight, so I used a 5/8" socket and ratchet wrench and hit wrench in counter clockwise direction to loosen the bolt a bit, then removed bolt using socket wrench, there is a oblong washer between the bolt and blade that comes right off, it has two dimples that fit into two small holes on either side of the blade's bolt hole.
Removed old chipped mower blade and "washer", then installed new blade and "old" washer onto "axle" that the bolt screws into, making sure the "washer's" dimples are aligned with the two small holes on either side of the new blade's bolt hole.
Finger tightened the bolt, then I tightened the blade bolt fairly hand-tight, then I researched what the required torque necessary for the blade to be safely tightened,
using a torque wrench, I tightened the blade bolt to 42 ft. pounds. which is right in the middle of what's required by my Troybilt manufacturer's specs.
I change out my air filter, spark plug, and blade about once a year with each growing season.

replaced the worn blade with a new blade

Removed the old blade and replaced with the new one, which has done very well.