T-Shirt to Workout Tank in a few Snips! Pretty self explanatory on 'how to' in the pic.
Then, you can tie it on the side or in the back depending on how baggy you prefer your tanks. OR you can cut a slit from the bottom going up the seam a little ways on the side and you have a homemade tie…. kind of like that Forever21 red tank I posted a while back.
OR, just let it be.