clean both threads of so they are really nice and there is no resistance when screwing them up, then clean the internal tapers and the external tapers and make sure you dont get anything on them that could make them slip. start the nut up the thread then hit the top of the joint with a hammer to get a good contact between the 2 surfaces of the tapers, screw the nut up and clamp the threaded part with some mole graps once it appears behind the nut.