This is an excerpt taken from Charcuterie from Scratch by Tim Hayward
Purchase the book here.

Note on sausage (skins) casings
; Sausage skins, or ‘casings,’ are traditionally made from cleaned lengths of the digestive tract. There are obviously some fairly stringent methods employed to clean what is effectively a pipe full of poop.

Lengths of gut are turned inside out, scraped to remove the soft lining material and repeatedly washed. What’s left is a tough membrane, impermeable to liquids; edible, though without any flavour; that shrinks as it dries.

You can order casings from your butcher or any online suppliers. They will arrive salted, vacuum-packed and probably in ludicrous lengths. Trust me, there are few more satisfying sounds than that of 130 feet (40 metres) of pig gut landing on your doormat. First thing to do is unpack and sort them. Cut them into manageable lengths of about 3 feet (1 metre), and then repack them into smaller batches and freeze. They’ll keep indefinitely in the freezer.

To use a batch of natural casings, unpack them and soak in several changes of clean water. This will remove the salt and make them softer and more manageable. There’s no polite way of describing the next bit. You need to pick up the wet membrane and slide it on to you sausage horn like a wet sock, wrinkling it up so you can pack as much on as possible. You’ll probably be able to load it up several metre lengths.
Different gauges of sausage casing some from different parts of the digestive tract and from different animals. Medium to large size, from the large intestine of a cow or pig, will work well for fat bangers and salamis, chipolatas will need the small intestine of a pig, and merguez is usually done halal-style, using the small intestine of a lamb.

A haggis, monstrous boiled delight that it is, uses ‘ox-bung’, probably the least euphemistic euphemism ever for the very last metre or so of the cow’s alimentary canal.