Since this is knit bottom up and in pieces, it's a lot easier than when it starts with the hood. Do you want just a neckband, or a foldover collar? If you want to do a neckband, pick up sts with the RS facing you and just work a few rows of ribbing.
For a collar, follow the hood instructions for picking up sts except pick them up with the WS facing, and you may want to do the one increase row. Continue in pattern or ribbing until it's the length you want and BO. The collar will flip over the neck and hide the pick up ridge.