Take your well fitted tank top and figure out where to cut it so that it will be below your bust, making an empire waist
Make the cut, discard the bottom half and set aside the top
Now for your T shirt, it should be a fitted shirt, but still a little loose. Cut the shoulders off just below the neck line. Also cut off the sleeves at the sides. (Sorry I don't have pics for this part, I don't know what I was thinking) Next sew up the sides where you cut off the sleeves and then pin the cut bottom part of the tank top to the cut top part of the shirt and sew.
And here's what you get! Pretty fast and easy :)