You might like to start with a pattern for insulated lined pants like this: http://www.jalie.com/sewingpatterns/patterncovers/958.html

You could then sew in channels for holding the down in place before completing the side seams.

I found down terrible to work with. I would strongly advise wearing a dust mask while stuffing the pants.

Just as a matter of curiosity, why down rather than a more easily handled and more easily washed insulation?
_________________________
Kate XXXXXX
Mad sewing witch!
http://www.katedicey.co.uk