I sewed a line of straight stitches down the center of the turtleneck collar and a little ways past, then I zig-zagged over the stitching.
Then I cut between the stitching.
This is what the back looks like after the collar is cut down the center:
Using white crochet thread, I did a single crochet edging all the way around.
I found this stitch in an old instruction book for crocheted bedspreads. I "borrowed" the border stitch of one of the spreads for my sweater.
The completed crocheted lace: