Do not remove the couplers, as there is no need. Remove the large screw that the short cab bogie pivots on. Then remove the 6 screws under the trailer floor. The black chassis can be separated from the brown floor of the trailer to access the guard's compartment for decoder installation. Remove the guard compartment floor (brown) with integrated weight as room is required to fit the decoder, speaker and stay alive. The model comes with an 8 pin socket for a decoder. Plug the decoder in, but align the grey decoder wire with the white mark on the socket (usually the orange wire would align with the white mark). Refit everything ensuring any live connections are carefully insulated. Reassemble model and test.