Define what sausage you want to make; i.e. breakfast, italian, sweet, polish, bratwurst, etc.
Do you want the sausage made as fresh sausage or smoked sausage?
Do you want breakfast links in sheep casings (size of your little finger) or dinner links in hog casings (size of two fingers or a large thumb)?
Be glad to help but need to know particulars; sausage is a huge field with many many variations and complexities
Adding additional fat depends on how lean the meat is to begin with and what texture/flavor you're going for; again, specifics.