Like CANADIANJOSH ,
Tighten as tight till the nut does not move freely .
Lock Tight ... I myself never use it , but I guess a fellow could use the red ( NOT the blue - you'll bust something BEFORE you ever get it back off again - )
You could also do what I did on one of my engines , go to you local hardware store and get a nylock nut that will fit the shaft . ( I brought mine right into the store to make sure I got the right size )