Step 1: Breast basics
+each breast is 90% water, can weigh 200/800 gms, sits on the chest wall between the 2nd and 6th rib,
+statistically, left breasts are often slightly larger than right ones, (up to 2 cup sizes!) though i encourage anyone to challenge this by submitting your own research on the topic.
+breasts have no internal support structure and rely almost totally on skin tension for their shape. this is where the bra comes in.
Step 2: Making your pattern
as with many things, the best way to learn is by taking a good bra apart. (its also possible just to reproduce the measurements of your current favorite bra . be exact; its deceptively easy shave off a few millimeters here and there and come out with a very odd looking bosom.)
Step 3: The underband
the underband of your bra is critical for fit, support and comfort; it cantilevers the weight of the breast into the lower back, making a secure platform for the weight of the breast.
(keep in mind the length of the bra underband is smaller than the natural rib cage measurement of the wearer-- a common underband length for a 34 band bra is 24.5". The formula fit for a 34 band size has a 29/30 rib cage measurement-- therefore the underband has to stretch from 24.5"-29" when worn.