This is the original, bottom up version, worked from the bottom up. For a top down, seamless, version, see this Ravelry download.

ERRATA IN THE MAGAZINE:
If you are knitting size 12, please start the body on row 3 of chart A (same as size 8 and 10).

I have made a video tutorial here about how to knit a tidy and strong buttonhole using crochet cast-on method which may be useful for this pattern ;)