The first thing I did was lay it flat and make the neck hole larger:
Next I turned it over and cut a V into the back and a straight line down the center. Then I cut from the straight line in the center two inches towards the sides all the way down on both sides, about two inches apart:
I tied those pieces together, which causes the back to gather. The results look like this:
If I wasn't sick and exhausted I would style this up with a cute tank or bralet underneath. I have a band t-shirt that I did this with that ended up being much tighter. So the results may vary depending on the shirt. To see more images and description of the process please visit my blog post about it here!