My method is as follows:
I multiply the desired diameter of the stuff sack by 3 (techically pi, but it's a stuff sack after all) and add 1 inch for seam allowance. So, a 4" diameter stuff sack would require 4x3+1 = 13 inch wide material.
I then cut the material the length of the finished stuff sack plus the diameter plus 1 inch for seams. So, 10" stuff sack would be cut 10+4+1 = 15 inches.
Next, I fold the material in half so that it measures 15"x6.5" in my example above. I cut one end of the doulbed fabric in a semi-circle shape. This will be the bottom of the stuff sack.
I then unfold the piece and sew the draw-cord in along the 13" side by folding over about 1" of fabric.
Finally, I double the fabric again as previously described and sew the one long seam that goes around the semi-circle bottom and up the side to the draw-cord end.
As a bonus to tackling DIY projects, one usually ends up with about 3 new stuff sacks!