Measure loosely around the fullest part of your bust, with the measuring tape right across your back, bringing it to the front. This is your bust measurement.
Wrap a measuring tape around your natural waistline, making sure to keep the tape parallel to the ground. This is your waist measurement.
Wrap a measuring tape around the fullest part of your hip, making sure to keep the tape parallel to the ground. This is your hip measurement.
Slip on your favourite pair of jeans, the ones that fit you best. Take your measuring tape and carefully measure the length from the crotch seam to the bottom of the leg. This is your inseam measurement.