Sewing or hemming a shirt collar might seem complicated, but it's actually rather easy. As long as you have an iron and some sewing pins to help you, the hem stitch is very simple and does not require the skills of an expert seamstress or tailor. This technique can be used if you are making a shirt from scratch or simply tailoring it to fit better.
Use the scissors to snip the fabric all around the collar, about every 3 inches. The cuts you make should only be about 1/2 inch long.
Fold the collar under 1/4 inch and iron it in place. Fold it under another 1/4 inch, pinning it down as you go.
Sew a straight seam through the middle of the folded-under part of the collar. Iron one last time.