By taking these two measurements, you can easily find your bralette cup size. The BCD is the most important. This is the distance from the bra bust point straight down to the bra wire line measured on the true vertical. You must wear a reasonably well-fitting bra in order to take this measurement.
The cross cup measurement is taken from the wire line on the outside to the wire line on the inside. If you aren't wearing wires, the measurement is taken from the seamline on the side of the cup to the seamline of the cup on the inside. Take the measurement across the bust point and try to keep the tape level with the floor. Then use the chart on the pattern back, or the one here to find your cup size.
For example, if your BCD is 4.25 and the CC is 9.5, you would use size H cup as shown below in this chart
Go to the back band pattern piece that corresponds with your rib cage measurement. You can choose any of the 6 band lengths and they will fit any of the 12 cup sizes. If you are in between band sizes (the bands are sized in even numbers, so if you measure 31" on the rib cage instead of 32" for example), I suggest you choose the longer and shorten it after you try it on, as the band is the easiest thing to change. So as you see, I would trace the H cup, shown in pink for my band.
If you need a band longer than 38" (96.5 cms), it is a simple matter to lengthen the band after you trace it out. Here's how: slice the band pattern piece from top to bottom parallel to the Dirction of Greatest Stretch (DoGS). Now draw an "H" on a separate piece of paper with the lines of the vertical being 1" apart (if you want to make your band 2" longer, you need 1" on the pattern piece) and draw the horizontal line roughly level to the DoGS line on the original pattern.
Pull the pieces apart and put a piece of paper underneath, lining up the horizontal line with the DoGS and the vertical with the edges you cut. Lastly connect, the top and bottom edges of the pattern pieces by starting at the side seam and drawing a line to the centre back area as shown.
You may notice that there are two different lines at the hook and eye area. The lower line, marked in blue is for a 2 high (called 2 x 3) set of hooks and eyes, while the pink one is for the 3 high hooks and eyes (called 3 x 3)
So now you have your bralette pattern sized up and ready to cut out. The hardest part is now deciding which of the 16 variations of the Sweet Sixteen you are going to make!