I'm trying to finish off the 'Cecily Beanie' from knitting daily.com
and I've got to the end after all the decreases (this hat is knit flat) and now the pattern says
"Break yarn, leaving a 12" (30.5 cm) tail. Thread tail through rem sts, pull tight to gather sts, and fasten off inside.
With MC threaded on a tapestry needle and beg at gathered top, sew seam using mattress stitch or backstitch (see box), reversing seam for lower 3⁄4" (2 cm) for rolled brim"
I am confused because it doesn't tell me anywhere to cast off! Do I cast off before or after the gathering?
I am not familiar with this technique of gathering and then 'fastening off inside'