I got this pillow case set from a thrift store a few months ago and never found anything to do with it, then I got a bunch of men's white shirts. It was destiny.
a white t-shirt
two pillow cases (or any other fabric you like)
and a sewing machine
Step 1: Preparing the Pillow Case
Rip all the seams and sew it into a long panel. Make sure it'll fit the widest part of your thigh, if it won't do this then you'll need more fabric. Sew all of the rough edges down.
Step 2: Sewing the Pillow Case
First, you'll need to measure where you want the end of the shirt to hit you, if it's too long, hem it up. Next, pin one of the corners of the pillow case onto the bottom seam, pin the rest around and cut off the extra fabric. Sew up the side hem and where the shirt meets the pillow case.
Step 3: Extra Decorating
I decided to cut the neck and the sleeve hem off the shirt off. I'm a big fan of cut edges on t-shirts so I didn't sew it back, but I'm sure there are plenty of things you can do to the shirt to make it more fun: tie dying, adding belt loops, cutting up the shirt, etc. Enjoy!
If anything was unclear, please let me know.