Cutting up the shoulder and fat
The pork shoulder you get at the supermarket tends to be a bit lean - they trim the excess fat off, maybe leaving a thin fat cap (which is always on the hidden tray side). You can ask them for the shoulder with more fat, but you're going to pay the full price for the fat, which seems a bit silly, but is an expedient solution. Tell them you're going to be grinding it for sausage. For good sausage, though, you want more than 20% fat, so having a source of fat you can add is a good thing. I have several pounds of nice pork back fat from a CSA in the freezer, and I use that to boost the fat content.
Bone-in pork shoulder is a lot cheaper than boneless. The bone-in was $5/lb, and the boneless is $6 or $7/lb. As it happens, out of this 4.75 lb (76 ounce) roast, the bone was 10 ounces, or about an eighth of the total weight. Your mileage may vary and "retail price" for the meat tends to go in 1 dollar steps. In any case, you need to cut the bone out. Next to the bone, you'll sometimes find a sort of gloppy soft fat, as well as the sinews that connect the muscle to the bone. You want to trim those out of your meat - they don't help the sausage, and will gunk up the grinder.
After removing the bone, it's time to slice it into appropriate sized chunks. I like chunks about an inch on a side, so I start by slicing the roast into 1" slices, and then cutting up those slices. There's also a picture of some gorgeous back fat which I've cut into roughly 2" cubes and wrapped in wax paper before freezing. It's much like cutting up butter. The whole fat/meat ratio is something you sort of wind up doing by eye. It just looks right.
This tray goes in the freezer to chill the meat and fat. That keeps it from melting during the grinding process.
Getting the seasoning together
Up to this point, pretty much all sausage is the same - you want that same ratio of fat/meat. For this batch, I'm making Andouille, using the recipe from Rulhman's book. I typically want it a bit spicy, but not lava - so when you see the recipe, adjust the cayenne and pepper flakes accordingly. Andouille uses some pink salt which turns the meat a reddish color as it cures/cooks in the smoker. This andouille recipe has the dry mix plus diced onions.
And here we are, with the meat cut up, fat cubes added, and seasoned. Back into the freezer while the grinder gets assembled.
Preparing the grinder
I'm using my trusty Kitchen-Aid grinder, which has two grinding plate sizes. For this, we're doing a single grind through the fine plate. Other sausages (like breakfast sausage), you might do two grinds, one through a coarse, and then a second through the fine. Or, if you want distinct chunks, the coarse might work for you. Here's all the pieces of the grinder. The two plates are on the left, and the cross shaped thing is the actual blade that does the cutting. I throw all the pieces in the freezer while I'm working on the meat, so that they are chilled before grinding.
Put the auger into the housing.
Blade on the end of the auger.
Put the Appropriate plate (fine, in this case) on top of the blade.
And you put the nut on the whole thing and tighten it down.Now, were ready to grind. I put the bowl into a bowl of ice cubes to help keep it cold.
And away we grind
You set the mixer to speed 4 (the second click) and start feeding the cold meat into the hopper. I keep a mixture of fat cubes and meat, and just keep it going.
At the end of the grinding process, I run a couple pieces of paper towel into the hopper, and that pushes the last bit of meat out. Here's a picture of the auger after disassembly, with the paper towel wrapped at the end. 
After grinding, it's important to mix the forcemeat thoroughly - this helps develop the internal structure and makes it hang together. The grinding process cuts all the muscle fibers, releasing a variety of substances, and you want things like myoglobin to make the meat stick together. That's also why you need salt. When it's done, it has a distinct sticky feel and uniform look. Some sausages don't need quite this much mixing - if you overdo it, you're making an emulsion, like in hot dogs or mortadella.
Stuffing
Stuffing the sausage has 3 steps - getting the casing ready & loading the stuffer; then actually stuffing; and finally making links.Getting the casings ready
The casings come in roughly 10 foot lengths, with a bunch of them in a hank in a plastic bag with lots of salt. They might well be shelf stable at room temperature, there's a lot of salt on them, but I do keep them in the refrigerator. I separate out 3 strands from the mass (which always seems to get all tangled up, like kite string), and put the remainder away.
You soak the casings in water for about 20 minutes, after rinsing the outer salt away. This is really important - the salt has extracted all the water from the casing (and preserving it), but you need to put that water back in, so that they are flexible and elastic again. I fill the casing with water and drain it 3 times to rinse any salt from the inside of the casing.
Yes, the casings are made from pig intestines (at least these casings are - you can also get sheep and beef casings which are smaller and larger respectively). There are "non-animal" casings made of cellulose, and I started with those - they don't work well - they're not as stretchy and strong, and that's really important when you start stuffing them. I've had really good luck with the hog casings - The only time I have them break is if it's damaged, or I accidentally poke a hole in it. They're roughly the size of your finger and will be about twice that diameter after stuffing. (somewhere I have a picture of the kids making balloon animals with inflated casings).
Preparing the stuffer
I have a nice 10 pound stuffer with a geared crank handle to run the piston down the cylinder. My largest batches are 5 pounds, so it works pretty well. There's a whole range of spouts you can put on for the various size casings. There are also horizontal stuffers with a sort of lever arm that pushes the forcemeat through - I've not tried one, but acquaintances who have say the vertical stuffer is better and more controllable - gear drive helps.I started trying to use the stuffer nozzle on the Kitchen Aid - that was horrible - the hopper was very small, and you can't start and stop easily. With this stuffer, you can put the entire batch into the cylinder, and all you have to worry about is cranking the piston down. A motorized stuffer is going to need a foot switch, because you need at least one hand to hold the casing as the forcemeat squirts in, and a second hand is handy. With a hand crank, you crank with one hand, and hold the casing with the other.
You start by putting the casing over the spout - I have a baking sheet with the wet, rinsed casings on it, and then, I start feeding it onto the spout, until it's all loaded.
And then it's crank with the right hand, hold the casing in your left. I try to make the coils neat and not kink it. The water on the pan helps things slide around.
Making links
"linking" is one of those things that gets easier with practice. I try to make my links about a handwidth long. You grab the filled casing, squish it with a thumb and forefinger on either side, and then sort of twirl the casing to twist the link ends. You wind up basically doing every other link this way, and the twists are opposite on each one.I then tie short pieces of string around some of the twists, because I'll need that to hang it in the smoker.
Chilling after smoking
After the sausage comes out of the smoker, you cut the links apart (the twisted part of the casing is all dry and hard now) and dump them into an ice bath to rapidly cool it. I would think that any pathogens inside the sausage are dead (heat and nitrite and salt), but there will still be things on the surface. And you don't want to put hot sausage into a cold refrigerator.Spicy Italian
I'm using the recipe from Ruhlman for this, too. It has paprika in it to give it that distinctive red color, along with toasted coriander and fennel seeds, along with fresh oregano and basil.
For the Italian sausages, they're a fresh sausage, not cooked. After stuffing, I put them in the refrigerator for a few hours so that the surface of the casing dries. Once the casing is dried off a bit, they're ready for vacuum packing and freezing.
