Feel for the top of the fundus/uterus(which will be towards the top of your belly/stomach). Measure with measuring tape, from that point down to your pubic area.
You uterus is measured from the top of your pelvic bone to the top of your uterus. Also called your fundal height, in centimeters this should correspond to how far along you are ie. 12 centimeters at 12 weeks pregnant.