Hooooboy!
That's got to take some sort of prize for newbie question of the year.
But don't worry about it, we don't judge (mostly) and there are lots of people willing to help. The short answer is that you get a cable that runs from your source and terminates in so-called RCA connectors that you plug into the inputs on the Crack.
In the picture below, the RCA inputs are in the top-left corner of the Crack (the two gold bits sticking out of the top plate).
(https://forum.bottlehead.com/proxy.php?request=http%3A%2F%2Fwww.bottlehead.com%2Fsc_images%2Fproducts%2F442_large_image.jpg&hash=9a8f9f48974ebdeb900483c80c2fbe7bf0cfbecb)
Depending on your source, you may need a 1/8" mini-phone to RCA cable (e.g., for use with some computer sound cards or a portable audio player), a 1/4" phone jack to RCA cable (e.g., for use with a television), an Apple 30-pin connector to RCA cable (for use with an iPod/iPad/iPhone) or an RCA to RCA cable (for use with some computer sound cards, most external DACs (digital-to-audio converters), CD players, etc.
For a picture of an RCA cable, see below:
(https://forum.bottlehead.com/proxy.php?request=http%3A%2F%2Fwww.bluejeanscable.com%2Fstore%2Faudio%2Faudcombo.jpg&hash=9a1363c214aac820d3498b3f9e8eec97e0681395)
Best regards,
Adam