The diagram with the sweater pattern is actually quite helpful. Follow that to keep yourself oriented. I think the pattern is missing a line, added in italics below. After you cut the yarn, you'll have the back yoke on a holder and the right front yoke bound off. Now start the left front yoke with no sts on the needle by casting on 18sts. It's like starting a totally new piece. You'll work these 18sts for 3 rows, put the back yoke on the needles and then work the back yoke & front yoke sts separately until the join row.
Next Row: K18 (for right front yoke), slip 18 sts to a holder (for back yoke); leave yarn attached to back yoke for later use.
Working over 18 right front yoke sts only, k 3 rows. Bind off the 18sts and cut yarn leaving ~6 inches to weave in later.
LEFT FRONT YOKE
Cast on 18 sts (for left front yoke).
K 3 rows.
Join Left Front and Back Yokes