i shot a meat buck this year for the sole purpose of turning the whole thing into breakfast sausage. i've done it with duck breast also. by far the best way to eat a duck IMO.
anyway here is what we do- soak it in salt water at least over night, then cut it 30% with strait pork fat or 40% if it's pork trimm.
one thing we noticed about the % equation:
cutting your mix 30% is NOT 10 lbs of meat and 3 lbs of fat
30% is 7 lbs of meat and 3 lbs of fat.
you end up with 10 lbs and 30% of the package is fat. at least that is what we talked are self into. it did seen to turn out better.
as far as the seasoning, you can buy it from any meat cutting place, you'll have to try a different brand each time until you find the one you like the best. and when you grinding and mixing it up, through a sample patty in the frying pan, and see if you want more fat or more seasoning.
i use all my elk meat for steaks and burger so it's nice to have a bunch of breakfast sausage to make. for ducks i cut them 50%. they are real dry.