Measure the circumference around the lower portion of the neck (Just under the Adam's apple), where the collar usually sits.
Measure the circumference around the upper (largest) portion of your chest, just under the armpits. Try to inhale when measuring this will ensure that your measurement account for your chest at its fullest.
Measure the circumference around the fullest part of your waist. Just above the belt line, and this measurement is almost always larger than your pant waist size.
Measure the circumference around the fullest part of your hips, just below the hip bone.
Measure accross the back, from one edge of the shoulder (shoulder seam if the shirt fits perfectly) to the other. Make sure that you take into account the curved contour of the shoulder (i.e. do not measure across in a straight line).
Measure from the base of the neck (where the shoulder meets your collar) to the desired length.
Measure around the circumference of the largest part of your bicep. Let your arms rest at your side and do not flex your bicep.
With your arm at your side, measure from the tip of the shoulder to 1” below your wrist bone.
Measure this for each sleeve to account for any differences in length. If this is for a short sleeve shirt, measure to the desired length, just beneath the bicep is typically preferable.
Measure the circumference around your wrist (at the bone) where a watch would typically be worn. If you wear a large watch add 1/2 inch to the measurement.