Sewing on a shawl collar like this is a little more like sewing with fabric than most seaming in knitting is. I would have the sweater facing me right side out an would lay the collar with the wrong side facing up on top of the sweater with the edge of the sweater and the neck edge of the collar touching each other evenly.
Then I would sew them together with as narrow a seam as I could, probably with an overcast sewing stitch using the yarn you made the sweater in or whatever yarn you sewed the other seams with if the sweater yarn is not appropriate for seaming for some reason. Have the center of the collar in the center back neck and the fronts where they are supposed to be and if they gave you a point to match at the shoulders that is nice, otherwise do the best you can to get collar eased into the space it needs to cover. After you are done you fold the collar back into its position and it will cover the seam.