If the button you're using is 4 stitches wide, make a 3-stitch buttonhole (as demonstrated). For a 5-stitch-wide button, make a 4-stitch buttonhole, etc.
Step 1 -- work to where you want buttonhole to begin. Bring yarn to front; slip 1 purl-wise, bring yarn to back.
Step 2 -- Slip 1 purl-wise, pass first slipped stitch over second. (repeat this part 2 more times--passing over 3 stitches total, for a 3-stitch buttonhole) Place last stitch back on left needle. Turn work.
Step 3 -- Cast on 4 stitches (for the 3 stitches you bound off, plus 1) using the cable cast-on method.
Step 4 -- Turn work (so RS is facing). Bring yarn to back; slip first st of left needle onto right needle and pass last CO st over it. ....You're done! Work to end of the row, or to the next buttonhole!