Hi, yes, garter st is knit all rows, and a purl ridge is formed when you knit 2 rows. So you knit 4 rows then on the next row, work a buttonhole. Mark that side with a safety pin or piece of yarn as the RS.
Could you please edit your post to remove most of the pattern? It's a copyright violation to post an entire pattern here, even if it's free; the site owners can get into trouble. It's okay to put a few rows that you're having a problem with, and a link to the pattern though.