the two sets of sts to be cast off will usually have been kept on a holder earlier in the pattern. Transfer these onto two needles and hold parallel with RS together.
insert a third needle of the same size into the back of the first st on the front needle and the front of the first st on the back needle. Wrap the yarn around the RH needle and pull through both sts.
rep Step 2 in the next sts on front and back needles then pass the first worked st on the RH needle over the second and cast off. Rep to end and fasten off.