The cover is actually almost square, it's just skewed in the photo because of the boning and the sofa.After I saw a few of my friends using a nursing cover, I got the idea to just make one myself. Realizing that this was just a big rectangle with straps, I set out on an internet search to discover dimensions and what made the "stiff neck" piece (corset boning). I took some inspiration from the covers I found on Etsy - Nursing Covers. And found a step-by-step instruction here - Breastfeeding 123.
Being one to never follow a pattern word for word - I altered the size of the cover for two reasons - the size she gave was too big for me. Also, I wanted to use the fabric I had left over from the diaper bag, which has a definite pattern and turning it sideways looks funny. Mine is approximately 24"x 26".
I also didn't like having the back of the fabric exposed and since both my fabrics are lightweight, I decided to stitch them together and flip right-side out, rather than just hem around the edge of one piece of fabric. I tend to have straighter edges this way also. Plus, this means the only visible hems are at the top edge. I did one line across the lower edge of the straps, under where I inserted the boning, and one across the top of the fabric to confine the boning and add double support to the straps.
I also cut the straps way down. I didn't like the construction of her straps, so I made straps the same as how I made them for my bag, only without any added "stiffening" fabric or interfacing. The tutorial I use for straps is here - Sew, Mama, Sew.
Since I had everything on hand except the corset boning - I spent only $1.36 for this cover.