Just did this three times (cause I had to do one of the rears twice). As suggested, slide the cap on first, then tighten the screw, then slide the cap down over the screw and catch in place on the metal clip that is under the screw.
The key to sliding the cap onto the armrest is to squeeze the armrest and slowly work the cap up the arm rest a little bit at a time. You need to squeeze all the way around and slowly push the cap up while compressing the arm rest material under the cap as you move it up the arm rest. You really only need to move it up a few centimeters, just enough to get a narrow Phillips head driver at the screw (at a slight angle). Don't try to get the cap so far up the arm rest that you can get the screw straight on because it is not necessary. As rsporsche suggested, a little soapy water may help, though it may make it tough to get a grip on the arm rest to push under the cap edge as you slide it up.
Good luck.
