I've watched some videos on EgoPowerPlus.com and YouTube and read my mower manual several times about how exactly to remove and sharpen the blade.
I put a screwdriver into the small hole to stop the blade from turning while loosening the nut. It wasn't super easy but I removed the blade. The washer fell to the floor but I picked it back up. It may sound silly but I wanted to try out removing the blade before I sharpen it.
I then put the washer back, ensuring the flat side is aligned with the flat side on the mower.
I inserted the blade and tightened it up. The "this side toward the lawn" is facing "down" or towards the grass. I start the mower and there is excessive vibrating and a bit of additional noise. I took it off and checked the "balance" of the blade and it was dead on. There was really no need to check as I didn't do anything but take it off and put it back but I felt like it anyhow.
I tightened again paying attention to how tight it was and it's pretty solid (I tried a couple levels of tightness and of course there are varying degrees of additional vibration and noise but still there.
What could I have done wrong?
There's a washer, a blade, and a nut. Boy do I feel inadequate!