There are a couple ways of doing it which depends on your board revision. If you could tell me what revision you have, I can point out the pick-up points for a pre-amped signal.
If you can't, or don't want to wait, the easiest way is to just run the lines from the headphone jack solder contacts, then you can use the slider to adjust volume for the output. Some say this is not as "clean" as the pre-amp signal, but I don't think there really is much of a difference. It's just a matter of preference. I like this method better since I can get more volume without having to mess with TV remotes while playing.
Thank you william and jeff, jeff, I don't know which revision I have, can you please tell me how to check it. Thank you very much. I tried to do it from the headphone output, but the sound is not very clean. Hope you can help me. Thank you very much.