you’ll need to measure the shirt-ee for match -OR- you’ll be able to simply wing it, and soak up an inch or so at a time.
measure in a number of locations, to get an concept of the scale you need.
VERY IMPORTANT: flip the shirt inside-out.
write down the measurements;
say, waist:32 stomach:28 bust:30.
now take every of those numbers and divide by 2.
so, waist:16 stomach:14 bust:15.
now measure these out on the shirt, and mark along with your sharpie.