You're going to bind off 5 sts, knit until you have 8 sts remaining and put them on a holder. You'll now turn your work and bind off 3 sts, purl to the end. You should have 5 sts left to bind off now, which is what you want to do.
Now you will work with the sts on the holder. If you can knit directly from whatever you used to hold them, do so or move them back to the needle. Make sure you have the RS facing you, attach your yarn and cast off the 1st 15 sts (they're in the center of the piece) and knit to the end. You'll then turn again and bind off 5 sts, purl to the end. Turn, bind off 3 sts. You should be done.
I hope I got that all right, if not someone will correct it for you.