Insert the trim piece on the armrest and carefully slide out of the way. Attach the armrest using the two lower screws, but do not tighten them fully yet. Picture shows upper screw already attached, please disregard.

Attach the upper screw and tighten down. You may need to flex the armrest slightly to align it.

Tighten the bottom two screws. Make sure you tighten these down well, without stripping them.

The armrest screws get a lot of use, so re-check them after a few days to make sure they haven't loosened up.