If the fabric is open (like a loose knit) just insert your hook into it and start making stitches. If it is too tight, sew a blanket stitch around the neck. Then treat the blanket stitches like a foundation chain.
I just did that very thing on a cotton sweater. It had a huge cowl neck that seemed to take over my upper torso--I unraveled the collar and used the yarn from it to re-do the neckline. I made a chain stitch close to the raw edge and crocheted a shell stitch edging onto it. It looks fabulous, if I do say so myself.