This diagram gives you an idea how a simple NMEA 2000 works. You can see the power node will connect to the battery and it's recommended you put a switch on the hot side so the network can be turned off.
On your MG Xi5 , there will be a NMEA gateway kit out shortly that will connect from your trolling motor to the NMEA backbone. Your HDS unit also connects to the same backbone as described in the diagram.
Brian