Last week, I figured out another (hopefully simpler) way to make the drawstring closure on a yoga bag. Here's the bag I made today using this new (and hopefully improved) method.
I started with a piece 19" x 8". 19" is the circumference of the yoga bag. The other dimension should be at least 1/3 the circumference. 1/3 of 19" is just over 6". So, at 8" we're safe (the concern is that if this second dimension is too short, the drawstring closure won't be able to close nicely).
First, sew the shorter sides together, right sides facing, to make a tube. Press the seam open.
Fold the tube in half, wrong sides facing, and press to make a crease along the middle of the tube.
On the seam, mark one point 3/4" above the middle crease and another point 3/4" below the middle crease.
Stitch a rectangle around the seam between those two marked points. Unpick the part of the seam inside the rectangle. This will be the opening in the drawstring casing.
Fold the closure in half, right sides out (the same way you did to make that middle crease) and stitch all the way around the tube, 3/4" from the fold to make the casing.
Pin the raw edge of the drawstring closure to the outside of the the yoga bag opening, and stitch with 1/4" seam allowance.
Press the seam closed towards the inside of the yoga bag (this is easier to do if you turn the bag inside out first).
Fold the top of the yoga bag down to make a ~1" hem at the opening of the bag. In this case, I made the hem 1-1/4", but if I were to do this again, I would use a 1" hem to leave more space inside for the yoga mat. Stitch the hem down about 1/8" above the drawstring closure.