Fold the top of bag 1/4" toward the outside and press. Fold over another 1/4" to encase that raw edge and press again.
Make the drawstring casing
Turn and stitch
Turn the sack inside out. Push out the bottom corners (I use the point of my scissors for this) and lay seams as flat as possible. Stitch again, a little more than 1/4" from the edge, making sure to entirely encase first seam.
Turn the bag right side out again, so that the encased seam lays on the inside of the bag. Push corners out.
Fold in half and stitch
Fold in half, wrong sides together (if your fabric has a wrong side) to get a squarish shape. Stitch the two sides, starting at raw edge and moving toward the fold, about a 1/4" from the edge.
Cut out the pieces
Cut a lot of them. I did mine in a variety of sizes, but the vast majority were made from 25” x 13” swatches of muslin. I also did a few that were a bit longer, to accommodate unwieldy things like lettuces and chards. Just remember to cut pieces about twice as long as your intended bag length.