The best way to fit your Bstring is to attach before wearing your bra. Once your bra is on, you can adjust Bstring as required.
Try different styles of bras to see which works best with your Bstring. Higher cut bra's may work better than balconette styles.
The better your bra fits, the better the Bstring fits.
You can leave Bstring attached to your bra to use for next time.
Always remember to hand wash Bstring separately.