it's much easier to do if someone else does it for you, rather than trying to do it yourself.
(this is from hearing midwives trying to do their own fundal height!)
you measure from the top of your uterus - so, walking the fingers down until you feel that "ledge" that is the top of the uterus. Then, you measure down to the top of the pubic bone.
Typically, after 20 weeks or so, it starts to somewhat correlate your weeks with the centimeters, give or take 2cm or so.