I'm not sure this qualifies as a tutorial since the explanation in the video leaves much to be desired, so I'll break it down further here...
The method is simple... Disconnect the wires running from the amp to the speaker and connect them from the amp to the common lugs on the switching jack. Then run wires from the other side of the jack to the internal speaker. Voila! Just remember, the tip is positive (+) and the sleeve is negative (-).
Mod safe and mod awesome!