It's a very simple construction, basically a pillow that is filled about halfway with flax seeds, and sewn shut. So, with the two rectangles right-side-together, I sewed three sides of each rectangle with a 1/2" seam. I turned them right side out, and pressed the unsewn edges under (to the inside of the pack). Then I topstitched around the three sides I'd already sewn at about 1/4" from the edge, for added reinforcement (flax seeds are small; you don't want them slipping out between your stitches!).
The image my friend shared as a sample showed channels, so I followed suit and made 2" channels, sewing from my topstitching line at the base to within 1/4" of the open side. I think the channels help to distribute the weight and warmth of the flax evenly across the pack, and prevents overstuffing, so that the pack is flexible.
Then I used a funnel to fill the packs, one channel at a time. I only filled each channel approximately half-full, so that the pack would be flexible. I pinned the top of each channel shut right above the seeds. When I had filled a full pack, I took it to my machine and stitched the open side closed, stitching 1/4" from the edge so that it would match my other topstitched edges.
These are made with almost the last of one of my favorite fabrics, a toile-print cotton canvas (I read that tightly-woven fabrics like canvas were preferred for these). I guess you just warm them up in the microwave and they hold their heat for hours. All in all, the cost to me was about $4 for a big bag of flax (I didn't quite use it all), and I suppose I'll wind up paying $5 or $7 for priority shipping - and I used up more of my stash of leftover fabric scraps.
My friend asked how she could repay me and I requested a surprise in the mail. I am excited for this exchange! I love barters, trades, and sending delightful little gifts in the post -and who doesn't love to get a package? It's always like Christmas, when that happens!