I assume you're picking up stitches along the front?
Work 2(3,3,3) stitches in the ribbing as established, yo and k2tog (or purl depending on where you are) Work 12(etc.) in ribbing, yo, work 2tog and keep doing this until you get to the end, then work 4(etc.) in ribbing.
The yo's will create the buttonhole and the k2tog will make up for the increase of the yo.