Keep in mind these are directions for getting the casings right and filling them. If you are looking for a good sausage recipe to stuff these with, try my recipe for French Toulouse sausages.
Assuming you have skin-on neck, plucked from just under the bird's head to where it met the body, you'll need to clean them. Push the meat and bone out from the skin. It will be attached by connective tissue. Use this for duck stock.
Turn the skin inside out. Pull off the trachea and the esophagus. Rinse under cool water, removing any bloody spots. Return the skin to outside out, and dry it on a cutting board while you do the next one.
Use your fingers to stuff the necks with loose sausage work from the fat end to the skinny end, leaving about an inch at the end. Use kitchen twine to tie off the skinny end tight and compress the sausage against it. Keep filling the neck until you have about an inch and a half at the fat end. Tie this off tightly, and use a sharp knife to trim excess skin.