First you need to measure the widest part of your body that will need to go through the skirt (usually hips)^^
Finished?
Now divide it by 2*3.14 ( twice pi ) and you'll get the needed radius.

Now fold your square of fabric in four. From the inner point measure the radius you've got. Make a few measurements and mark, so that you'll end up with quarter of a circle - as shown in the picture.
Cut on the line - you'll get a hole in the middle of your fabric square.
Do the same with the second piece of fabric.

Put the two pieces on top of each other matching the holes as shown in the pic.

Now fold the fabric edge around the hole inside (1/5 cm would suffice) and sew. Make sure you're sewing both of the pieces together. You'll get a messy edge folded inside.
Now, to hide the mess and prevent your skirt from falling apart - fold again, as the blue arrow shows, and sew. This time make it fit your elastic band's width.

All you've left to do is to insert the elastic band into the skirt!
First fit the band to yourself - put it around the place your skirt is going to be on, tighten a bit to make it hold and mark the needed length. Then insert it inside the fold you've sewn and sew it to itself following your mark.

Wear and enjoy!