I've done several of those switches so far.
Frist you make the hole the right size, in the place you want it to be.
With your switch cable of choice, making sure before hand, that the swich cable AND the pickup cable can fit in the hole from pickup route to lower control route. Enlarge as needed.
Drill a hole from the pickup route to the upper chamber big enuf for the switch cable
Optionally - you may also want to route a little trough in the pickup cavity for the wire that goes to the switch.
Pass the switch wire from the pickup cavity to the upper chamber and fish it out of the F/hole
Wire up the switch
Send a fishing line thru the hole you drilled for the switch, and pull it out the F/hole
Place the star washer on the switch, and tie/superglue the fishing line to the switch - with no plastic lever attached
Train, trick, beg, cajole, threaten, and or spank the switch into position using the switch cable and the fishing line.
Place the switch ring and nut over the line and start them onto the switch
Tighten the nut and remove the line - done
Optionally - you may want to make a little tool that fits thru the F/hole, to hold the switch from spinning. One I did with no tool, the rest I did with a little bent piece of aluminum about 3/4 inch wide and 1/16 thick and ... maybe 6 or 7 inches long. It just goes in there and wedges between the switch and the body. You gotta keep the switch position lined up, but other than getting the nut tight... its really pretty simple.