The top is knitted from bottom up, then the cups are knitted in a circle with the rib knit and the stitches are increased for the breasts according to the diagram until the short row knitting starts for one cup at the time, knitting back and forth as well as increasing every fourth row. Now you keep knitting the cups and the straps but you put the back and armpit St on another circular needle of the same size or a band ( with a needle) that you thread through the stitches to keep them safe.
Then when the cups are to the right size you bind off the top of the cups but you leave the straps along with one extra St ( edge stitch) that is a part of the cups. Then you keep knitting the straps and when you are close to the length you want,