The retrofit was fairly simple. VA-25 reportedly had all 12 of its aircraft modified midcruise in a couple of weeks at NAS Cubi Pt, Philippine Islands in November 1967 while Coral Sea was off the line. It basically consisted of a rocket located under the canopy behind the pilot's head and a different seat/headrest. The rocket was connected to two ten-foot lines that were attached to the pilot's parachute harness.
The rocket was installed to the right of the existing canopy actuation mechanism.
When the pilot activated the system by pulling a D-ring located between his thighs, the canopy was jettisoned and the rocket tilted upward and fired, extracting the pilot out of the cockpit. This is a test using a civil-registered T-6G.
Although light weight, the extraction concept was only appropriate at the relatively low speeds usually attained by propeller driven aircraft.
For more stuff on the Yankee seat, see:
http://users.bestweb.net/~kcoyne/frame_sg.htm (Click on the Stanley "Yankee" button well down in the left column )
For a comparison with the original seat, see http://tailspintopics.blogspot.com/2013/11/ad-1-skyraider-standard-vs-extraction.html