This is a pattern created to have almost as many sizes as possible.
You choose your circumference and knit the rib knit from the bottom up, choose your cup size according to your circumference ( cup sizes change depending on the circumference )
Then you knit your cup size according to the chart with rib knit around the back and under the arms until the last chart then those stitches are put on a circular needle or a string of yarn while you finish knitting the cups and also the straps.
Then you bind off the cup part and continue knitting the straps, I recommend trying it on to see how long you like the straps to be.

I recommend yarn that isn´t too stretchy the Merino DK that dottir dyeworks is selling is perfect.
You need between 1-2 skeins.