1) Remove coupler chain pin from coupler box on both ends of model
2) Remove 4 body mount screws and coupler assemblies from underside
3) Wiggle body vertically off chassis
4) Remove 21-pin dummy plug
5) Fit the decoder
6) Fit the speaker
Fit a 23mm speaker for mediocre sound quality, or
Fit a premium DCCSound speaker for far superior sound.
Option 2 requires making some cuts to the chassis with a Dremmel tool and cutoff wheel. The effort is worth it!
7) Test operation and then reassemble body back on chassis along with body screws and couplers.