It looks so simple - The Shirt Skirt...
I found a maternity t-shirt that I could use.
I bought elastic thread. I wound three bobbins with it, by hand.
I attempted shirring.
I failed. *sigh* I'll revisit it again, but it was frustrating.
So, instead, I put a narrow elastic waistband on the tube of the shirt and made a really deliciously comfortable skirt.. not what I wanted to do, but it was still wearable.