I love this macramé back bra I created. It's great solution to wear with those open back blouses and even as a beach or yoga top.
Just follow the macramé videos I did here:
I did a variation of these two:
I used strips of a tshirt but you can really use any type of string.
Simply cut the back of the sports bra off. Be careful not to cut too much from the side because the strings don't cover much. I'd mark where you want to cut while wearing it. Then pin and sew right sides together.
For the side make sure to spread the strings out to get them to fit the sides. Sew right sides together. That's it, the hardest part is fitting which is up to you.