How to tighten this screw?

[GoC]

Ahhh. You hold the innermost nut, while tightening with the outermost one?

No, just screw the nuts together by hand. Then use a wrench only on the outer nut. They will lock anyway. You'll notice how it works when you start screwing. But you should hold the inner nut when you begin to unscrew the outer one after you have tightened the screw.

 

[sinuslinus]

You can screw on two nuts on the thread or use the other part as a lever.

edit: seems like I was the last one to get it.

 

[Huddingebo]

Dome nut works too.