I'll start by mentioning that if you're worried about, it might be best to hire someone to do it. On average, you'll probably find that two holes (and running the wires) should be about 3-4 hours. A little more, depending on circumstances - limited access to the spot can certainly increase that.
That said, what you'll do is verify (a few times!) that you're cutting in the right spot. Take a piece of 3/4" MARINE plywood and make a backing plate to be epoxied on the inside. It should be roughly twice the size (square is OK) of the ducer. You can pre-coat this piece with epoxy ahead of time (and sand lightly, after washing the amine blush off). Now, sand the interior surface where the block will be epoxied - sand about an extra 4"-6" in each direction. Next, epoxy the piece in place and cover with fiberglass.
Now you're ready to cut the hole. If space permits, I usually like to make a pilot hole from the inside to make sure I get the right position. If not, measure a few times from known objects (keel, lifting strakes, etc.). Once your big hole is cut, bevel the inside and outside edge of the cut. I didn't realize the boat had a balsa core there, but in that case then what you want to do is remove about 1/2" of the balsa all the way around the inside perimeter of the cut (sometimes it might be easier to do this before epoxying the block in place) and then come back and fill with thickened epoxy (first coat the balsa and inside area of this "relieved" section with straight epoxy). Coat the inside edge of the blocking at the same time.
Seal the 'ducer with something like BoatLife LifeCalk or 3M 4200 (after again removing the amine blush and a light sand). DO NOT use 3M5200. Again... DO NOT use 5200.
