For the chest, lay the shirt flat and measure from the armpit from pit to pit. The measurements begin and end at the fabric, not the seams.
For the length, start at the high point shoulder (HPS) - this is where the shoulder meets the collar. Measure straight down the front of the shirt until you reach the bottom hem
For the sleeve, start at the shoulder seam and measure all the way to the end of the cuff.
For the shoulder, measure straight across the shoulders from sleeve seam to sleeve seam.