On the first row, you bind off 12 stitches, then knit (or purl, or whatever) the remaining stitches in that row. Then on the next row, you bind off the first 12 stitches and knit (or whatever) the remaining stitches, which should be 10 stitches. Then you garter stitch those 10 stitches until you have knitted 2 1/2", then bind off the last 10 stitches.
Since you knit back and forth, the first row will bind off 12 stitches on the right, then the next row will bind off 12 stitches on the left, leaving you with the 10 stitches in the middle of the row.
Mommy to two wonderful girls
"I don't have to conform to the vagaries of time and space, I'm a loony!" -Campbell Bain (David Tennant) 'Takin' Over the Asylum'
OTN: Cabled Sweater
, arm warmers, kids' clothes