Problem is, the pattern shows a whole lot of nothing. There is a diagram, but it does not demonstrate the clipping and folding which must occur. I desperately tested out different folds and tried to think of a backwards way to work mitered binding, but nothing worked. Then I turned to Google, but all I could think of was "attach double fold bias tape corner," which eventually led me to "double fold bias inside corner." Lucky for me, there is someone else out there who struggled with this pattern and I found some online help for sewing a "square neckline."
The responder to the original post recommends a book called Power Sewing. It helped, but it took awhile for me to understand what needed to happen and being the skimmer that I am, I could have used more photos with shorter explanations. As you can see below, I finally figured it out, and now I have my square neckline complete, with binding. I still have to attach bias tape to the side edges of the handles and finish the bag itself, but I am enjoying the sweetness of finally being smarter than the square neckline.
How I Finished a Square Neckline (a.k.a. Inside Corner) With Double Fold Bias Tape (With this particular pattern, I use "square neckline" interchangeably with "upper and inner edges of bag handles")
- Stay stitch the upper and inner edges of the bag handles. Stay stitching should be 1/8" from seam; so with double fold bias, stay stitch 1/4" from raw edge because the seam will be 3/8".
- Clip inner corner, almost to stay stitching.
- Right sides together, raw edges even, pin narrower side of the bias tape to neckline and sew until you near a clipped corner. The seam allowance is 3/8" and stitching should be in the fold of the bias tape.
- When you near a clipped corner, stop sewing, but do not remove the needle from the fabric. Carefully spread the fabric at the corner so you now have a straight edge. Pins help here, but while you continue to sew, be careful not to pinch the bag fabric, which cannot be seen under the tape.
- Press seam toward bias tape. The photo above shows a sample of how the corner will look once the bias tape and bag are sewn together.
- Fold at the corners, right sides together. Be careful the fold is right at the corner. Stitch across fold, but instead of a straight seam, sew a "V." The bottom of the "V" points toward the center of the bag.
- Fold bias tape over to wrong side of the bag. To finish the neckline, topstitch about 1/8" from open edge of tape. Topstitching is done on the right side of the bag fabric, and this is why the narrower side is attached here. Finishing can also be done by hand on the back if you are working with more delicate fabric.
I am not quite sure if this is supposed to be as challenging as it turned out to be, but I am hoping it is not meant to be easy since I did take sewing lessons from Mrs. Iki when I was a teenager; and while I do recall using bias tape for a blue-green tank top, I do not remember any encounters with square necklines. Hopefully this makes things a little bit clearer for other sewers out there. If you have any questions, just post a comment and I am happy to help. If not, then you are a better sewer than I am, and I wish I had your skills!