Measure the circumference at the place where you will actually wear your jeans. Your waist measurement will be the entire circumference of the area where your jeans will rest. So if you want a low, mid or high waist jeans you will measure the low, mid or high waist circumference and not your natural waist.
While measuring, position the tape very close to the body without tightening. Wear a belt or rope around the place where you want your jeans to rest so that it will also help you to measure your length and crotch. You can also use a marker to highlight your waist line where you want to rest your jeans.
Please do not provide the label size of the jeans.