Does anyone know the tool that helps stop the power steering from moving while tightening the nut??
Put a long screwdriver through one of the holes in the pulley. Brace the blade end of it against the body of the pump. Place it in a way so the pulley won't move. Ensure the end isn't against something which will break (like something plastic). Then tighten the nut. There really isn't (to my knowledge) any real "tool" to get this done. You just have to wing it.