When you look at the first picture I posted above (if the cap you have is the same), you'll see the two arms of the cap, each which end in C openings.
In the second picture, with the cap removed, you can see the pivot point, where the rear wiper arm can be elevated so it can pull up and away from the rear window. In that same picture, at the the top and bottom edges of the pivot point, where the pin allows the arm to be elevated, you'll notice a small gap, between the thin outer edge of the wiper arm, and the rotating arm base. The C openings of the cap clip in to that gap.
In the bottom pic, with the cap installed, you can see where the C openings of the cap arm have been inserted into that gap, and the cap rotated down and pressed into place.
From memory, with the wiper arm laying flat against the rear window, try holding the cap almost at a 90* angle to the wiper arm, such that the C openings of the cap arms are touching the gaps at the rear wiper arm pivot point pin. Now slide the C openings of the cap arms into that gap (shown above), and rotate the cap down and over the washer jet.
