Lay the shirt flat, shoot straight down (camera square over the shirt, not at an angle).
2 · Calibrate
Drag the magenta dots to the armpit seams. If you fill in the height too, drag the teal dots from the neckline seam to the bottom hem — it's used to warn you if the photo is at an angle.
3 · Design
Drag the design to position it. Pinch (or scroll on a computer) to resize — or type an exact print width above. The big number is what you print at.
4 · Export
Sheet × in
Leave mirror OFF if your print software already mirrors — double-mirroring is the classic mistake. When the design splits, each tile includes the overlap and small tick marks show the true seam: trim there and butt-join the tiles on the press (don't overlap transfers — the overlap double-presses and darkens).
PDF is the safe choice: pages carry their real size, so printing at "Actual Size" / 100% comes out exact. PNGs are for print software that wants loose images (transparency is kept; PDF flattens onto white).