Step 1: Starting out
The first thing you need to do is find the pin-out diagram to tell which pin handles what function. These things are available all over google for whatever port you might find.
What else do you need: Soldering iron, glue gun, continuity tester, shrink wire, port connector, female a/v jacks
Step 2: Understanding the pin-out
Here I'm going to use the six audio channels, the amplifier remote wire, and mute wire
Right now I'm ignoring the rear view camera and video hookups.