Got one of these from EBay -they are invaluable as most of the parts assemblies have quality line drawings.
Here is Opel's pic of the window winder:
#2 is the rubber washer; #10 the spring lock (as they call the retainer clip)
#11 is the plastic washer; #12 the Spring (conical) as they call it.
If you install the 'spring lock' in to the slots in the handle before fitting the handle it should snap into place as you push the handle on to the spline .. while holding all the other bits in place ..... never said it would be easy!
Whoa! Think there is another "Opel-strikes-again" in that drawing - looks like the spring lock lives in the recess on the plastic washer - maybe #11 should be fitted with the spring lock holding it on to the handle before the handle is installed ..... this will let the handle rotate using the wide flange of the plastic washer bearing against the #2 Washer (rubber) so that the spring clip does not get tangled up in the upholstery ... good luck!
HTH