Help with Audacity

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I've been asked to supply some bass for a band a few hundred miles away and I'm in need of some help and assistance as I'm a complete beginner when it comes to recording stuff.

I have a version of the song in mp3 format without the bass and given the distance it has been suggested that I record the bass part on a WAV file and post it on the web (the last bit I can do)!

My question is this; I can play the file using Audacity, but can I record the WAV file at the same time and listen to the results in real time. Also what is the best way to get the bass into Audacity? Can I plug the output straight into my laptop, or do I need to get the signal from my DI box (or from the back of my amp), and do I need to convert it to digital format first - I've seen these cables with a mic style plug at one end and a USB at the other.

Basically I'm Dazed and Confused. Please help me if you can