Summarize:

You can also purchase old shirts at a thrift store if you don't have any that need to be refashioned. The more worn the fabric is, the better it will work. High-low shirts tend to be drapey and hang loosely over the stomach and hips. They can even be layered over tank tops. Baseball shirts or those with a similar hem are even easier to convert.
Find an old t-shirt or button down shirt in your closet. Choose a shirt that is loose. Opt for a shirt that has a flap in front and back.