Separate the collar parts and sew on the machine, just the outside of the collar to the jacket. Then turn the inside seam line of the collar under, and press so you have a neat fold. Then blind stitch--sometimes called slip stitch by hand the inside of the collar to the jacket.

If you don't know how to blind stitch this is how. Attach the thread in the fold. Bring your thread out right on your pressed fold line. Take a small stitch in the jacket right on the sewing line where you sewed on the outside collar. Then into the fold on the fold line. Run the thread along in the fold a little bit, then out of the fold on the fold line, and take another stitch in the jacket. Your stitches are running parallel to the sewn seam line and should be small and even.