you will need to measure the shirt-ee for fit -OR- you can just wing it, and take in an inch or so at a time.
measure in a few places, to get an idea of the size you want.
VERY IMPORTANT: turn the shirt inside-out.
write down the measurements;
say, waist:32 belly:28 bust:30.
now take each of these numbers and divide by 2.
so, waist:16 belly:14 bust:15.
now measure those out on the shirt, and mark with your sharpie.