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 - http://en.wikipedia.org/wiki/NMEA_0183

Lowerance HDS 7 - http://www.lowrance.com/Root/Lowrance-Documents/CA/HDS_INSTALL_IM_EN_988-10043-001_w.pdf

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