It is based on this tutorial by verypurpleperson:
This bag is very cute but is slightly fabric-consuming. Bags are relatively efficient and don't waste a lot of fabric because they are usually made up of relatively square pieces, that pack well on fabric. Since the straps and the bag itself are cut as one fabric piece, this leaves a "gap" of pretty useless fabric pieces in between the straps. Obviously, matching plaids also wastes more fabric than usual.
The instructions call for reversing the bag through one of the straps - I initially found this a bit peculiar but I understand why it was done this way - reversing the bag through one of the other, rounded, seams, would theoretically be easier, but it may be harder to top-stitch the round seam from the outside later on. Use a sturdy fabric that doesn't get damaged or stretched out of shape when it is stretched by the pressure of trying to squeeze a clump of fabric through it!
An edge stitch foot is invaluable for getting precise, beautiful, even edge stitches. Don't set the needle too close to the edge, or the pretty top stitching won't be visible. Use a 3 mm stitch length for all of your top stitching, and a top stitching thread, which is thicker and more pronounced than regular thread. Top-stitching around that tight curve is tricky, so go slow and you'll be fine.
|my beloved and beautiful top-stitching|
|inner cellphone pocket, and snap|