Is the question "How do I connect them?"?
If so, the attached image may come in handy:

- Model-B-Plus-Audio-Video-Jack-Diagram.png (22.68 KiB) Viewed 1100 times
Since you have a stereo output and a mono amplifier, you need to output everything through one channel and hook that to your amp or combine the two channels. If you want to combine the stereo channels into one mono signal, put a 1000 ohm resistor in series with each channel and tie the other ends together. This page linked-to below shows the process, albeit not Raspberry Pi-specific.
https://www.neatcircuits.com/magic_cable.htm
I've done this with lots of audio circuits.
Cheers,
LightBike