Tubular Cast-Off for 1x1 Ribbing
Once you have completed the desired amount of ribbing, you can work a tubular cast-off (which looks identical to a tubular cast-on) as follows:
Step 1: As with the cast-on, work a foundation row by knitting each knit stitch and slipping each purl stitch. Repeat this row 3 times (you have worked 4 foundations rows).
Step 2: Now, using two spare circular needles (preferably smaller than the ones you were using), separate the knit and purl stitches as follows:
Knit Stitches: Slip the knit stitches to the front needle purlwise (so they don't become twisted).
Purl Stitches: Slip the purl stitches to the back needle purlwise.
Note: You can start with either a knit or purl depending on which is first.
Step 3: The separated stitches are now ready to be grafted together.