After you bind off the stitch, the one that's left is technically part of the second set of nine, I believe. So you'll have 9, a gap where you bound off, and 9 more. Is that the way it looks? Then you'll work each side of the gap separately for the buttonhole and join the sides again when it's done.
"Trust the pattern!"
Time flies like an arrow, but fruit flies like a banana.
Hop aboard while we follow the cloud-covered van of knitting harmony, man!! Woooo!! --Hedgehog
IngridKH on Ravelry