I am listing one of these skirts in my Etsy store.
Step 1: Choosing a T-shirt for the waistband.
Step 3: Making sure the waistband fits.
You don't have to fold the waistband, but it makes it more durable. If it's for a maternity skirt or if you prefer bunching it for style you can leave it a single layer. If you leave it a single layer you can use i as a dress or a top.