You bind off in pattern- knit the knitted stitches and purl the purled stitches, passing the stitch on the right over just like in a regular bind off. (be sure to keep it loose so the ribbing will stretch!)
So say it's KK PP KK You'll knit 2 stitches, bind off one, purl the next, bind off one, purl the next, bind off one, knit the next, bind off one.
Hope this made sense!
OTN: - Tree of Life afghan, socks, afghan for Dad, Tura Maeve baby dress, alpaca Winter Mountains scarf, Frida poncho