Step 2: Cut
This muff is essentially a long doughnut, a pillow with a hole poked all the way through, a straight cylinder, like a roll of paper towels. The size of the fabric and stuffing should therefore be a rectangle twice the size of the desired end product, with a little extra allowance for tucking the fabric in on the edges and for seams.
Although I gave dimension in the last step I did not actually make any measurement while I was sewing this muff -- I really meant it when I said I was a sloppy seamstress. I just threw my coat on the floor, and eyeballed the approximate size. Since my old coat had become quite thin and limp I decided to double it up, so I drew a rectangle with a chalk around 30" by 16"
Do yourself a favor and do not repeat my mistake: I sewed along my chalk line to avoid loosing all the feathers in my rectangle, but I forgot about the other side. The coat was so thin I didn't think there would be too many feathers -- boy was I wrong! I attempted to staunch the flow of feathers with masking tape, and ended up wasting much more time than if I had simply sewn another line, one inch from the first one, and cut between the two.
To measure the right size for the outer fabric I folded my lining in two, placed it over the fabric and cut another rectangle, leaving about an inch extra on all the sides to allow for the material to be tucked in and for seams.