As always, we'll start by taking some measurements.
Measure the drape. Take your measuring tape and measure from one neck point to the other, allowing the measuring tape to dip as low as you want your cowl neckline to drape. Record this length (A).
Measure from one neck point down to just underneath the armpit on the same side (B).
Measure the same point underneath your armpit to your center back (C).
Measure your waist and divide by 4 (D).
Measure the widest part of your hips and divide by 4 (E).
Measure the desired length of your dress from underneath your armpit (F).