First off, decide the size
of the finished stuff sack.
Let's call the finished width W and the finished length L.
The side and bottom seams will have 0.5" allowance.
There will be a drawstring tube at the top which will be a 0.5" tube of
double layer of fabric and 0.25" seams.
The raw material will have width = (W + 1") and length = (L + W/4" + 2")
The top drawstring tube requires the extra 2" of fabric.
The finished bag is 'squared off' by stitching across the bottom seam.
If you want a rectangular bag, divide the width by 5 or 6 (i.e. W/5" or
W/6") before determining the raw material length.
Check out the detailed pictures at Thru-Hiker's
"Make Your Own Silnylon
" to understand how the bottom 'squaring off' works:
I prefer to start by making the drawstring tube (as described below)
BEFORE sewing the side and bottom seams. This allows me to
the drawcord tube while the material is flat which I find less