step 1: get your mitts on a boxy tee, cut off the sleeves and the neck banding.
step 2: turn it inside out. on both sides, mark in about a 3/4"-1" from the outer top armhole and curve it into the bottom of the armhole. you can guesstimate the shape, but you must keep it smooth. the point is to take off a little more fabric at the shoulder points to make the top less bulky. match the front and back armholes of the garment til flush. now trim with a sharp pair of scissors.
step 3: see where the red arrows point? on both sides at this shoulder area, you're going to cut along the seam, opening up the garment.
(this is what you want.)
step 4: twist once only the front shoulder panel piece. the direction is up to you. pin it into place.
step 5: sew it up. i just hand sew it. takes less time than to set up a machine.
step 5: turn it inside out and this is what you get. it's now slimmed down at the shoulder/neck area, giving it a better drape and silhouette. what was going to be an awkward muscle tee is now a tanky-hybrid. now go liberate the other thought-to-be-useless tees in your closet. give them a second chance.