Shirt designs I converted to .svgs using Adobe Illustrator and then to .stls using Fusion 360.
Same concept used by RCLifeOn, but with jojo characters (and the bro-op dog).
MAKE SURE THE SHIRT (or other article of clothing) IS SECURELY ATTATCHED TO THE BUILD PLATE! If any of it gets dragged around by the nozzle while printing it'll mess up and look hella ugly.
Make sure the model is scaled to be the same or twice the height of your layer height.
Set your nozzle to be a little bit hotter than normal, maybe 10C higher. I used 210 for PLA.