Its not as esay as you'd think.... there are several principles that are obvious... only one device can be connected to a 'listening' port, but several devices can be connected to a 'talking' port. Each port has a specific speed, usually either 38400bps or 4800bps. Some devices have several ports, some can have different ports configured at different speeds, some can't. Different devices use floating grounds, some use the negative side of the 12V circuit... and the most complex one, there is a long list of NMEA 'instructions' (the messages sent out on an NMEA network), and you need to make sure that the right ones are available to be sent, and can be understood by the receiving device... most devices can understand a subset of the NMEA instruction set.
I have 3 instruments (wind, depth, log/speed), a VHF with AIS built in, a plotter with built in GPS, and an autopilot, all of which need info from one or more of the other devices.
Its a fun exercise, and in my case needs a further devices, an NMEA mux that allows several devices outputs to be combined into one single output... this needs a PC connecting to it to configure the ports.... it also provides, in my case, a USB port, that I can connect to a PC to use as a further navigation option.
I have a background in IT.... god knows how some people cope with it!
The one thing I can almost guarantee... my design won't work first time, and i'll spend a few hours debugging why!!!