After washing your shirt, and ironing if necessary, cut away all the pieces that you won't need for this project- collar, sleeves, front button bands, and back yoke. And in the spirit of Miss Julia Child who said, "Save the liver!" please save your buttons and bits of fabric that can be stashed for future endeavors. What you're left with will be some nice large flat pieces and you can fit a square template onto them wherever they fit in order to cut. A men's large dress shirt should net at least 5 napkins and you can slice open sleeves to get a couple more.
I happen to have a nice quilters template about 9 1/2 inches (but it should be noted that I don't have a serger! but I digress...) but you can easily use a cardboard template or just measure. A template is good if you plan to make a lot of napkins and want to cut quickly. I also use a rotary cutter (it's fun and I happen to have one - though I do not own a serger...woe is me.) A rotary cutter allows you to cut through 3 or 4 layers at once, improving your efforts - like a serger does when it comes to sewing... (There's no way my husband's going to read this and get the hint about the serger, so it's futile to go on about it ya know?)
If you do own a serger (I hate you first of all,) you can now simply surge all edges and you are done. I, on the other hand, will be moving to the next steps? Hmm. Let's proceed:
Turn each edge in about 1/4 to 1/2 inch and press all around. I like to turn my corners in first, making a little triangle in each corner and then turning the side edges under. This way you get nice mitered corners. But it's your call.
Using a zig zag stitch, buzz around all four edges catching the rough edge to prevent fraying.
Give a nice press, fold and stack and you're done.
I'm sincerely grateful that my very adequate sewing maching has a zigzag stitch. Some machines don't. My inner brat is now silenced and you are on your way to making eco-friendly napkins to carry to work and school. Rock on!