This book is also good for sausage-making and all things charcuterie.
Basic points, for a simple pork sausage: Pork shoulder/Boston butt has pretty much the right meat:fat ratio for a standard sausage. You want the meat pretty cold when you grind it. I cube my meat and put it in the freezer for a bit to harden up. Also, freeze all your sausage-making grinding attachments as well. The reason is, you want distinct pieces of fat, and room temperature meat will not grind cleanly very well, and you will end up with a sausage whose texture is off. (Add to that the fat will clog up your grinder and you will be getting mushed meat being poorly extruded rather than a clean grind easily flowing through.) If the grind starts looking sinewy and is bunching up on you, clean the blade and the grade. You do not want to me forcing the meat through a clogged up grate.
IMHO, collagen casings are terrible. I can’t find a good purpose for them other than throwing them into the garbage can. I always use natural casings (either the standard hog casings or hog middles for larger sausages.)
The Kitchen Aid stuffer attachment mushes the meat up too much for my tastes. However, it’ll do fine for when your beginning. I use one of these stuffers, which are great, as they just push the meat evenly straight through the tube, rather than having that rotating piece in there which gums up the meat a bit.
Proper salting levels are about 2-2.5% by weight for most sausages. Some go up to 3% or so, but you should aim for 2-2.5% and see how you like it. I use two scales for sausage making–one kitchen scale for the meat, and a gram scale for the salt and herbs. I do everything by percentages for consistency and to make easy scaling.
After you have it ground, that’s usually when you add the seasonings and the liquid. (Some people cube and season the meat first, then grind it.) You want to work the meat until you have a nice “bind” going. This is when the mixture becomes a bit tacky and holds together on its own. You can do this step in the mixing part of your Kitchen Aid, or you could do it by hand. It should take a couple of minutes to get the meat to this point.
Then, stuff in casings. This will take a little practice, but should be fairly self-explanatory.
There’s a reasonable video here with the basics.