Deano
Set up 3 of the stems in the cover into the adapter ring, have the adapter ring in the rear opening, then fit the 4th stem into the last corner and slide the stem into the last fitting from the side. You will have to keep a firm hold onto the adapter as you slide the stem home, the cover is very flexible. Hope this helps without a diagram, it took me a couple of goes to get it right, but think I have mastered it now.
Jim M