It's easy, just follow steps, 1-2-3.
1. Pick a shirt from the stack. If you like the color and the length, it will work.
2. With a sharp scissors, cut off the sleeves, at first just cutting them at the arm-hole/shoulder seam on each side. Cut the arm holes as long as you would like to (vertically). On the single layer of fabric in the front cut a line towards the ribbed collar, making the Bro-Tank straps about 2" wide.
3. Turn the shirt over with the back is facing up. Cut only the back layer of fabric from the bottom of the arm hole to the collar (in a C-shape), creating a t-back, while leaving the front full-coverage.