It took a little while to turn the edges and get them all the same, but as it is just going to be wrapped around baby I'm not too worried if it's a perfect square.
I started with it being 1m x 1m and then folded the edges twice. I did a straight stitch around the whole thing, about 1/4" from the top fold, to keep it all in place.
Then I changed to the contrasting thread and did a fancy stitch along each edge to catch that top fold and keep it flat.
Even though this will be a summer baby, I have a feeling I will be using this as a blanket a lot.
Edited to add some more pics of the process while I was making a second one.