An old collared shirt can be given a small makeover to create a new, fun style. It's easy: just add buttons!
I would recommend laying out your buttons on your collar before sewing them on, then taking a picture of the layout. I changed the layout a bit as I sewed, and added other buttons from my collection, but it was helpful to have a template to go off of.
When hand-sewing, I often think of a few tips by Alabama Chanin, who I had the pleasure of meeting a few years ago. Namely, that my thread should only be as long as my fingertips to my elbow, and that I should love my thread by running it between my fingers. Learn why, in this lovely video.
I also find it important to make sure to sew with a double strand of thread, and to make a good knot before and after I sew. Learn more about the physics of sewing by hand in this article, again by Alabama Chanin.
The good thing about sewing on top of a collar is that your thread is, for the most part, hidden from view. I started my stitches on the underside of the collar, but the majority of the stitching was made in between the top and bottom collar pieces. It gave a neater appearance to the top and underside, although I'm sure it could be neater if I had better sewing skills. Oh well :)