A. Fold down the edge of the bag (to the inside) so that the design on the bag is at the edge. Crease this fold. This particular bag has side creases folded in (like a paper grocery bag), so fold those in. Don't worry about the actual edge of the plastic yet, we'll tackle that later. Lay it flat on your cutting surface.
I don't advise using any types of pins, so when you need to have something held tight, use binder clips or large paperclips.
B. and C. I want 20" of bag from that top fold down to my bottom seam. On my cutting board I aligned the top edge of the bag with the 30" mark, and I aligned my ruler to cut at the 10" mark.
D. and E. The next step I did is to make deeper sides. Currently my birdseed sack would have a really wide front and back, but the sides would be narrow.
I start out by making my front right edge close to the design on the sack by making a new fold like along the design.
F. I repeat for the front left side.
G. and H. and I. Then I turn the sack over...and hopefully I can describe this so you can understand. I want the back to be the same width as the front.
With my new crease for the front of the bag and the original side center crease I make the bag lie flat so I'm creating a new side crease for the back that matches the front...and I repeat it on the other side.
Crease the sides well so that when the come 'undone' you can see where they're suppose to be.