Make sure you have the bigger and smaller washers the correct ends first.
Then fit the rear washer and put the rubber ring over the back and washer itself, use some grease on the surfaces and it will help mate them together and act as a glue to keep the faces together
You should then be able to offer the arm up and with the trumpet in one hand and arm in the other, feed them up. The washer should stay in place.
Make sure you have the long pivot and thick front washer handy so when the arm is in place, fit the washer in at the front of the subframe and then push through the pivot / pin. Put the plate on, then fit the small 1/4 in bolts in. The bottom bolt will need its 1/4in nut and lock washer handy. Tighten evenly.
The rubber ring for the front - Roll it over the front of the arm, if you try and leave it at the front, it will more than likely fall off on trying to fit the arm, so roll it over and push it up the arm a bit, then when the pin and everything is fitted and in place , you can push / slide the ring into place with a screwdriver.
This should help