This casting off method works with all rib stitches. It doesn’t matter if it’s 2×2, 3×1, etc. It will work for any of those.
Once you have reached the desired length of your project, cast off all stitches on the left needle as follows:
1. Work the first 2 stitches as they appear, this means knit the knit stitches and purl the purl stitches.
2. Insert the left needle through both stitches you worked on the last step and work them together the same way you worked the last stitch, this means, if you knitted it, knit both stitches together, and if you purled it, purl both stitches together (you will have 1 stitch on your right needle).
3. Work the next stitch as it appears (you will have 2 stitches on your right needle).
4. Insert the left needle through both stitches on the right needle and work them together the same way you worked the last stitch (you will have 1 stitch on your right needle).
Repeat steps 3 and 4 until there are no more stitches on your left needle.