I had a dingy old tee so I dyed it grey. Cut neckline, cut arms (as shown).

Gather the bottom hemline in a few inches to be snug on your waist (this is what changes the shape of the shirt). Sew the new sleeve seams you cut (everything is right sides together). Then you will sew a seam starting from the gathered end up the armpit. Make sure you sew the side seam going gradually out towards the edge to keep the fit loose. Then trim the excess fabric.