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Mark the spots on the pillow with a pencil where your chest, waist and hips are, according to the measurements you've just taken. This may be challenging to get through, but it is worth the patient effort required. The end result should be shaped a little like a human torso. This is to check that you've pinned the pillow in accurately. If the measurements don't match your initial ones, then keep adjusting with the safety pins. If it matches, move on.
Use the pillow as the body of the mannequin. Gather some of the leftover material on the pillow to adjust the pillow to the size and shape of your body, and pin it tightly into the shape of your measurements––take it slowly and do this with care, so as not to stab yourself with the pins. Once the pillow is adjusted, measure all three areas with the measuring tape.