All I needed was a yard of fabric, scissors, and various sewing paraphernalia (sewing machine and pins, to be specific).
The website I got this from suggests a jersey knit. I made a beeline to the "dance and swimwear" section of JoAnne fabrics and found a nice, stretchy cloth. Basically, if it won't fray when you cut it, you're in the clear. But cloth with nylon in it with make it more comfortable to wear.
Now, as you can see, my actual shirt's collar is very deep. What I forgot is that the "actual shirt" has zero give to it. This nylon blend? Lots of give. I tried on the fabric to see how the neck hole turned out, only to end up giving a free show to my cats. Instead of panicking, I moved forward with the project, all the while playing out how I'll fix this gaping hole for a collar.