I decided to knit the collar before the sleeves for this blue cardi. My reasons for doing this were fairly arbitrary, but if you're worried about running out of yarn (I'm not, but still), I would recommend doing this, that way you can use half of whatever remains for sleeve 1 and the other half for sleeve 2. The pattern instructs you to pick up 2 stitches for every 3 rows of knitting on the fronts, and 1 for 1 along the cast on edge at the top of the work.
I picked up the stitches, starting at the bottom of the right front, with the right side facing. I pick up my stitches between the edgemost stitch and the second edgemost stitch of the work, so that once the stitches have been picked up along the front, there's an entire column of (messy) edge stitches behind the row of picked up stitches
The way the stitches are picked up along the cast on edge, the collar appears to be continuous with the back of the sweater, which I like. This shows the collar along left front and sleeve:
Once you've picked up stitches ALL THE WAY around (this might take a while), you work back in forth in stockinette until desired width. I decided I wanted my collar to be about 4". I pondered how best to cast off quite a bit, and considered working a super small hemmed edge, but after a test swatch, decided against this method. In the end, I used a loose knitted cast off, worked on the wrong side but in knit stitches. I liked this better than the look of the knitted cast off worked on the right side.
Whatever cast off you choose, make sure it is super stretchy. It would suck to have a tight cast off on such a light an airy sweater. The collar is SUPER rolly, which is a little annoying, but I'll get over it. That's why I decided to go for 4", to compensate a bit for the mad rolling.
Next up, the sleeves!