Use a cloth measuring tape (you can buy one on Amazon here). Ideally, take the measurements at around the same time each week. Here is how to take the measurements:
Chest circumference
Measure the circumference along the widest part of your chest, along the nipple line. See images below.
Waist circumference
Measure the circumference along the narrowest part of your waist, in line with the belly button. See below images.
Thigh circumference
Measure the circumference of the widest part of your thigh. See images below.