what you need:

1 t-shirt or tank top that you won't miss. it has to have a straight/unisex cut (when you lay it flat on the ground, the sides go straight down, no curvy shape). good to have: the fabric should be good enough to still last you a couple of years, and you'll like some breathability as well.

a strip of jeans fabric or other stiff fabric that's somewhat breathable. you'll want a rectangle that can cover your entire chest area.

basic sewing supplies: a sewing machine, or a needle and thread and more time.

scissors (fabric scissors are recommended)

pins

STEP 1

if you're using a tank top, go to STEP 2.

cut the sleeves off the t-shirt. i prefer cutting just outside of the shoulder seam, that way you won't need to finish anything or worry about the fabric fraying.

STEP 2

decide what you want to be visible on the front of your flattener. the top's outside, inside (to get just the colored fabric without the print) or the strip of jeans?

if you want your top's outside, turn it inside out now.

if you want the inside, make sure it's currently the right way around.

if you want the stiff fabric, make sure to sew it on on top on after STEP 6 rather than putting it inside in STEP 4.

STEP 3

fold up the lower edge of your top *around the top* until it reaches the armholes. try to make the side seam, if present, line up with itself. if you managed to get it aligned symmetrically, the new lower edge should be horizontal.

STEP 4

pin down the folded up part. start at the armholes. then do the back.

stick your stiff fabric inside the front pocket you've just created (unless you want it on the front). if it's too long to fit inside, now's the time to cut it.

finish pinning.

STEP 5

start sewing. i like to do one stitch down the front and each side to keep the stiff fabric in place. then just follow the pins.

STEP 6

turn inside out. if you didn't put your stiff fabric inside, now's the time to pin it on top and then sew it.

STEP 7

wear! actually, it would be smart to try it on a few times mid-process to see if it's coming along well. but yeah, if the top is wide enough and the fabric stiff enough, it should lead to a flat shape! layer with wide clothes for the best effect! not recommended for swimming, just because it will definitely ride up.

i don't know how well it will work at larger sizes, i hope that someone can try it and share their experience!