A friend of mine once wore something like this and i thought I would try and make it myself. Once more... please excuse the flaws...I'm new in the craft-world, still have a lot to learn! My original idea was to convert a T-Shirt into a top, but as I have no sewing machine and only little experience it turned out so difficult, I had to settle for the old top. You will also have to excuse my english (it is not my mother tongue and this is not always helping me be exact). Hope you like it, it is for sure very easy to make :-)
Take your old top and tree (long and fat!) stripes of tulle.
Cut out the strap(s) of the top! You can optionally leave this step for the end. I was planning to cut both of them, but I finally cut only one and I did it at the end, cz it was easier for me this way.
Wrap each tulle-stripe around itself.
Make a pigtail out of the tulle-stripes. The pigtail should not be of full length.
Stitch the beginning of the pigtail, so as to "hold" it together (this way it will not unfold).
Stitch the beginning" of the pigtail on the back of the top, where you want your strap to begin.
Stitch the end of the pigtail on the front of the top and then stitch one of the big buttons on it.
Stitch the remaining tulle-stripes on the top. Here you go! Upgrade done! You can optionally replace both straps. With the colors I chose for the tulle-stripes, I thought it would be too much.