Chest: Lay the garment flat and smooth out any wrinkles. Start your measurement just below the armpit seam and measure straight across to the end of the fabric.
Shoulder: With the back facing up, smooth out the fabric. Start your measurement at the top of one shoulder seam and measure straight across to the other seam.
Sleeve Length: Pull out a sleeve and lay it flat. Measure from the shoulder seam to the end of the sleeve. If there is no seam (as on a raglan sleeve), do your best to estimate the edge of the shoulder.
Body Length: Lay the garment flat with the backside facing you. Measure straight down from the highest point of the shoulder (usually where the collar meets the body) to the bottom of the hem.
Hem or Bottom Opening: With the shirt laid flat and wrinkles smoothed out, measure across the bottom of the shirt from end to end.