I found this great how-to on runwaydiy.com and made this cute little collar in no time. I sewed most of it with a machine and then finished it by hand. I found it a little difficult to draw a perfect circle from the string and would recommend using a circular object like the base of a small pot next time as guidance.
I was going to threw out pile of old ill-fitting shirts. Im happy I did not do that, now those shirts have a new life.