Here's how to do it:
1. I took the sleeves off and chopped the shirt in half an inch and a half below my waist.
2. I took the top half in to fit my bust and waist.
3. I gathered the bottom half until it now was the same size as the top half and then sewed it back onto the top half.
4. And then I replaced the sleeves!
.... and yes dance parties spontaneously break out around here.