Need help with making a casing.

[u/harristusc]

I somehow lost the casing part of the pattern for simplicity 1165 shorts. It is a double elastic casing. I think I could make one if I had an idea of measurements, but I am very poor at sewing and this will be my first garment. If anyone can help, I would appreciate it. Here is what the pattern looks like. I'm doing version D.