Lay your t-shirt flat, back side up. Measure about 2.5" down from below where the sleeves meet the sides. (This is what worked for me - you may want to try the shirt on you to make sure this is the area where you want the corseting to end.) Mark.
Measure below that mark 1.5" and make another mark at each side. Repeat twice so you have 4 marks total, evenly-spaced at 1.5" apart.
Hand-sew the hooks on at each part.
Since we're sewing onto the back and the t-shirt fabric will be pulled to the front, make sure the hooks are facing outwards.
Try the shirt on. Take the elastic cord , fold it in half, and match its center to your bellybutton. Keep these lined up, thread either side of the cord into the bottom hook on either side of the shirt.
Pull to the front, pulling the shirt fabric with it.
Cross the cord over at your center, then back into the next hooks above the last.
Continue to lace the corset-front in this manner,
and rearrange the extra fabric of the tee as you go to look neater and more tucked-in to the corset area.
Tie the ends of the cords at the top of the corseting in a bow; trim the ends.