Well, If you've installed subs and amps as you say, you don't need detailed instructions on that part. Install as usual.
The stock HU doesn't have RCA outputs, so you can do one of two things:
1) Get an amplifier that has high level inputs. Simply split the rear speaker wires, and connect to the amp directly using the high level inputs.
2) Get a line out converter. This is a $20 device that basically takes speaker level signal (Again just split your rear speaker wires), and gives you a low level RCA output that you can connect to your amp...