There is a diagram on page 6 of the NGW-1 User Manual that illustrates the connections from the NGW-1 to an NMEA 0183 device. This same diagram can be seen below.
To clarify these instructions:
The red and black pair are the NGW-1 Listener pair (RX) for connecting to the output of an NMEA 0183 Talker (TX).
The blue and white pair are the NGW-1 Talker pair (TX) for connecting to the input of NMEA 0183 Listener (RX).
The diagram below provides a good example of how the NMEA 2000 network should be designed and how the NGW-1 fits in between your NMEA 0183 device(s) and the NMEA 2000 network.