Lay the garment flat. Measure from the lower points of the armhole seams across the chest. Then double the measurements for total circumference.
To figure out your pant/waist size, measure around your natural waistline, keeping the tape a bit loose.
Starting at the middle of your neck, take the tape around keeping the tape loose.
Measure from the center of the collar to the shoulder seam. Then measure from the shoulder seam to the sleeve opening. Now combine both to get your measurements.
Wearing pants that fit well, measure from the crotch seam to the bottom of the leg opening.