How to log data from my depthfinder?

I want to be able to press a button on my Arduino and start saving NMEA 0183 data from my Lowerance HDS 7 fishfinder to an SD card. The fishfinder has wires for TX +/- and RX +/- and uses RS-422 serial (page 31 of the PDF below). I am a beginner and how would I go about doing this?

NMEA 0183 data -

Lowerance HDS 7 -

You can buy an RS422 to TTL adapter and read the fishfinder output directly with the Arduino, via the serial port.