One of my favourite projects this summer was this granny square cocoon shrug, (while I didn't get around to writing about it at the time, I did post a pic to Instagram). I had wanted to make one for a while but wasn't quite sure where to start until I saw this version.
My square measured about 105cm across, and I used two strands of plötulopi (unspun Icelandic wool) crocheted together with a 7mm hook, which gave it a chunky but light texture with a nice drape. I also did one round of double crochet as the last round of my granny square.
Next, I folded it in half and joined the short sides from the bottom edge to the "armhole" (leave a space before the fold) on either side (I used a sc join).
For the collar, I started at one of the joined corners and added five rounds of dc all the way around, turning on each round to avoid an obvious "wrong" side when it's flipped over. To make "sleeves" (more like cuffs, really), I did three rounds of alternating dc and front post dc stitches around the armhole openings.
I thoroughly enjoyed making this, and I really want to try making a version that falls a bit more like a cardigan, like this beautiful piece by Jo Storie. In the meantime, I'm pretty happy with this one, and look forward to wearing it now that fall has arrived!
EDIT: I received a lot of questions about this, so I've written a follow-up post with an FAQ and step-by-step description. You can find it here. Happy making!