If I was attempting this, I would cut out material for two pairs of pants, one in my size and one size larger. I would make sure the length on each was 2-4" longer than I needed, just in case. Then...
Sew each one independently (but don't finish the waist band or pant hem).
Turn one pant inside out and put the smaller one inside the other, so the nice sides face out.
I would sew the top of the two pants together, below where the waistband will go.
Then I would start stuffing the pants with down, adding enough for about 1" of thickness and about 5" wide band.
Then I would pin down that band of material, trying to keep the inside pant flat and the outside pant would be puffy and curvy, which will take up the extra material.
And I would keep doing this for the entire length of the pants.
I think that should work ???
Another option would be to make and stuff each of the four quarters of the pants (front left, front right, back left, back right) and then sew them all together... I'm not sure which would be easier.
I can almost guarantee that if you can make a pair of basic pants, you'll be able to do this... it might not look completely professional, but it will keep you warm.
Now, that said. I wound up buying a pair of montbell down pants. I figured material and time costs would be too much for me. The pants are only about $100.http://www.prolitegear.com/cgi-bin/prolitegear/montbell_down_inner_pant_sp_07
Anyway, hope this helps either way you decide to go!