With RS of sweater facing you, start at bottom left.
PU stitches every 2 out of 3 rows.
Mark off the stitches at the neck corners with a safety pin. Purl those stitches. Every other row, M! on either side of the purled stitch.
After three ridges of garter stitch (6 rows), add a one row buttonhole:
Add another 3 rows of garter st. Cast off in purl.
Shawl collar with short rows: