I remember making something like this a pre-teen, but I think this is a little more refined.
These are my lonely pillow cases. You can also find discount ones at TJ Maxx or the clearance section at any home store. The one I use bellow I've had since I was little and now I have a new purpose for it!
Lay it out and cut about an 1/4 inch of the bottom where it closes and then cut at a diagonal.
Open it up to make a triangle and fold the 2 sides of the triangle in twice, pin and hem
Next fold back in half, right side out and stick one inside of the other
Lining up the bottom edges, pin and sew in front where the fabric overlap. Repeat on other side.
Turn the fabric inside out and hem the cut bottom close.
After, turn back right side out. To finish you can tie it closed or sew ribbon or toole like I did at the top to tie closeI use mine to take books to library and I've gotten a lot of compliments! People don't believe it when I tell them it's an old lonely pillow case!