The two arms of a dual pivot caliper are called the "C" arm and the "Y" arm, due to their respective resemblance to the letters.
The "C" arm is the one with the off-center pivot bolt and the cable anchor bolt. The "C" arm does not need to be disturbed to install a drop bolt.
The "Y" arm is the one that pivots on the center bolt. It also has the housing stop/adjusting barrel assembly.
Sequence of Operations:
Unhook the return spring using from the "Y" arm. A small screwdriver is convenient for this. Shimano dual-pivot calipers use a nylon anti-friction block, don't lose it!
Remove the 13 mm nut on the back of the caliper. Shimano units require a 12 point box wrench to fit the special 12 point nut.
(This nut will be discarded, and replaced by the block of the drop bolt.)
Loosen the setscrew underneath the center bolt, using a 2.5 mm Allen wrench. This setscrew presses against the threads fo the centerbolt to prevent it from turning.
Remove the Allen bolt from the drop bolt assembly and set it in a handy place.
Unscrew and remove the centerbolt from the caliper, using a 3 mm or 4 mm Allen Wrench. As you withdraw the center bolt from the caliper unit, hold the "Y" arm in place against the rear block of the caliper so as to avoid misplacng any washers or bearings that are between the "Y" arm and the block. Immediately thread in the new pivot bolt in through the "Y" arm into the threads of the rear block.
Adjust the center pivot bolt so that the "Y" arm can pivot freely, but not wiggle back and forth. When you think you've got it right, tighten the 2.5 mm Allen setscrew _lightly_ against the center bolt. Double check the pivoting action of the "Y" arm, and, when satisfied, tighten the setscrew snugly against the centerbolt.
Set the spring into position, with the original washer on top of it, and screw the drop bolt block down on to the projecting part of the centerbolt. Tighten it down with a 13 mm open end wrench. Ideally, it will be pointing straight up at 12:00 o'clock. If it doesn't line up that way (and the odds are it won't!) unscrew the drop bolt block from the caliper, and add some of the thin shim washers provided to adjust the angle. Once the drop bolt block tightens up snugly _and_ lines up at 12:00 o'clock, tighten the 2.5 mm Allen setscrew in the bottom of the drop bolt block to secure it.
Note, it is important that the drop bolt block be firmly screwed against the rear block of the caliper. Try different spacer shim combinations until you get a good snug fit with the correct angle.