[Steeveedee]

Do you have a factory service manual? There is a description in there on how to do this. Right out of the manual:

If you use a wrench to tighten the nut, spin the wheel and snug the nut, then back off 1/4 to 1/2 turn to get to a place where you can insert the cotter pin.

If you do it by hand, snug the nut and then back it off until you can get the cotter pin in. It should not even be left finger-tight.
There should be .001" to .008" end play.

You can't rely on the washer feeling tight.