At first I just started cutting and hoped for the best, needless to say that didn't work out so well. But thankfully you can sew basically anything back together. This is what I did that worked, and it turned out to be super simple.
First cut off the sleeves. Going along the seam is the easiest way to make sure you don't cut more than you want to.
Next, we all have that one tank top that fits which is why we wear it too often. Turn the shirt you're fixing inside out and put your tank top over the shirt, outline it and sew. Remember, don't sew your arm! just sew up to them.
This way is easier than what I tried at first which was to sew in a little from every side. Also this way you have a template to go by.
Tada! I think I've caught the fever. Now every shirt I look at I think how can I redo this?