I love that nursing cover but being on the other side of the hemisphere, I couldn't justify paying $60 and shipping just for a simple rectangle of jersey knit. Actually, I don't even know if they ship here.
I bought about 1.5m of stretchy lightweight knit fabric - I don't remember the composition exactly, but I believe it was a mix of things, nothing fantastic - and using the measurements on the website as a rough guide, I marked my rectangle, cut it out, hemmed the edges (folded it just once over) and zigzagged it. I'm no pro, I pretty much winged it, but prior I made sure to practise on a spare piece, playing around with the settings on my sewing machine till I could just roar through without any bunching/stretching/mistakes. I'm surprised at how easy it was really... the lady at the shop was warning me how difficult it'd be to sew with, but once I got the settings right I literally ran the whole thing through in under 10 minutes.
Thinking I had an awesome machine, I made another, but I messed around with the settings in between and it wasn't the same lol. Oops.
For the neck hole, I folded the fabric in half and in half again, marked roughly 5" from the centre (neckhole should be around 10-12" shoulder to shoulder), and cut just a very narrow strip off. It has to be very narrow for it to work as a carseat cover - I found my first one to be too loose around the neck (I cut about 3cm). The second one (which I preferred) I cut it around 1cm thick. Better to start thin, test it out, and if it's too narrow for your liking you can widen it. To 'finish' it I just zigzagged around the neckhole (no folding over).
Again, I stress I'm no pro (as if you can't tell!!) but hey it survived the washing machine fine (cold machine wash, line-dried) and it ties around my carseat fine so I consider it a win?
Photo of nursing cover: https://www.craftster.org/pictures/showphoto.php?photo=565910&ppuser=312812
Photo of nursing cover on carseat: https://www.craftster.org/pictures/showphoto.php?photo=565911&ppuser=312812
Rough measurements guide I used: https://www.craftster.org/pictures/showphoto.php?photo=565912&ppuser=312812
Rough guide on how I did the neckhole: https://www.craftster.org/pictures/showphoto.php?photo=565919&ppuser=312812