This is where my "trash liners" called HOME for several years...
21x25 in piece of fabric
1/4 in elastic cut at 11.5 in and another one at 6.5 in
18 in piece of ribbon
First things first. I taught myself how to sew about 2 years ago. I can't follow a pattern and I don't know sewing lingo, I figure things out that I think will work and just go with it.
First sew a casing for your elastic on both ends.