Use a flexible measuring tape
Measure chest around the fullest part, under the arms
Measure neck around the base where a collar sits
Measure shoulder width from one shoulder edge to the other
Measure sleeve length from shoulder seam to wrist
Measure shirt length from the highest shoulder point to the desired hem
Measure waist around the narrowest part of the torso
Keep the tape snug but not tight
Stand straight and relax your posture
Compare all measurements to the shirt size chart