I think my Dr has been accurately measuring me since about 16 wks. You have to be lying down perfectly flat on your back with your legs out straight. Then you take the end of the measuring tape and place it at the very top of your pubic bone. Run the measuring tape up and feel for the fundus (top of your uterus). That in cm's is how many weeks you should be approximately. For my first I always measured bang on for cm's verses weeks. For this pregnancy I am 2 cm's ahead. My OB said this is normal for consecutive pregnancies though.
Good luck, finding the fundus is sometimes really hard to do unless you are a Doctor or Midwife and know exactly what you are looking for!