To install the armrest, the best option is to remove the seat from the car. This gives access to the two screws at the base of the rear panel. The panel comes out by sliding it downwards and out.
The right side of the frame has all the holes for the armrest installation.
The armrest itself has quite a few components to be able to install ( as in the reference picture below )
The most PITA job is to fit the lower bolt from the outside inbetween the frame & seat upholstery. If that isn't hard enough, the worst part is tightening it into the metal arm which itself can't rotate since it has a locating pin that slides into the frame ( seen under the larger top hole ). You need to have access to a small open end wrench (size 10) that can fit in the small place ( there is hardly any space to rotate a wrench and it takes quite a while to fully tighten the bolt into the metal arm ).
Once the lower bolt is tight and the arm sits snug to the frame, you have to slide the spacer inbetween the frame and upholstry ( the same way as the lower bolt ), only this time, you have to align the spacers two holes correctly which can be achieved by sliding the spacer's lower hole over the arms locating pin. This is again quite a tough task since you have to force the spacer against the seat vinyl so there is enough space between the spacer and the arm's extended locating pin so that the spacer can slide over it.
When the spacer is fixed, you move to the most heart wrenching moment. Cutting a slit in the seat vinyl to slide the armrest bolster through it and into the frame and metal arm. The original vinyl has no pre-made cutting template for the armrest install, however, the seat covers that I have on the car do ( since these are 201 specific covers ). For that reason, I only cut a small X in the original vinyl just so that the armrest bolt could slide in. A longer cut might be necessary in order to cover the metal ring, that has a hole for the locating pin to slide into. I smaller second cut was made for the locating pin. The seat covers had the perfect template for the metal ring to snug inside and VOILA, the task is complete.
The armrest has two positions when in use. A single push of the button lets the armrest fall into a flat horizontal position, a second push of the button lets it fall further another 10 or so degrees.