Take any t-shirt you barely wear or buy a cheap one at Walmart.
Cut off the sleeves, 1 inch away from the armhole seems. You want the armhole to be decently wide because you will be tying it in the back.
Now, cut a deep neck, cut both the front and back layers at the same time.
Next, cut the back even deeper than the front, this is to reduce any pilling at the front and by the arms.
Last, put the shirt on and tie it in back with ribbon. I find it easiest to tie when it is on to ensure you tie it in the right place. If you desire, cut the bottom of the shirt in a curved fashion to make it cropped with small points on either side, this makes it great for layering over other tanks!!
Try this with all of your old t-shirts!!