Need help with sewing on collar

Hi, I just finished knitting a sweater and used mattress stitch for the sleeve and side seams, but this sweater has a separate collar. It overlaps in front and then fold down around the neck. I finally understand what they meant by overlapping the front seams, but now I am unsure of what kind of stitch to use to sew it together. Since the edges are finished, I can’t use kitcheners or 3-needle BO. Do I just whip-stitch it or will that be too bulky? Anyone have advice? Any help would be appreciated. THANKS!

Some helpful seaming info here:

Depending on your stitch orientation, check out some of their other info, such as