Fold the shirt so that only the sleeves overlap each other.;
, Take a Sharpie tracing on the edge from the bottom of the shirt all the way up to the collar. When you're done and you unfold each side of the shirt, it should form a column in the middle of your shirt.
 Your shirt should now have a strip missing. Put it on like you would a jacket and see if it's a good fit; determine how much you need to cut off to make it tighter or show more skin.
 You're shirt should be a little bit looser than you would like it. Cut off equal portions of the shirt without cutting the collar, so the strip is still connected to the shirt (unless you are using different string and not the shirt fabric, cut the strip all the way).
 Each dot should have another dot across from it, this is the corset pattern. Make as many holes as you desire, but don't cut one into the collar.
 Try not to make it too big, or too close to the edge, because it will be easier to rip and won't last as long.
 These are the strings for your corset.
 The strips may not be long enough, so you will need to sew on more fabric from the middle part you took out.

Lay the shirt on the ground with the front touching the ground, the back part where you're going to make the corset should be facing you.
Fold the sleeves back so it forms a straight line.
Cut only the back part of the shirt, cutting on the outer edge of the Sharpie line so that it's not on the good part of the shirt.
Cut off equal portions of each side of the shirt until it's a desired fit.
Momentarily move the strips and take your Sharpie making dots equal distance from each other on each side of the shirt.
Take your knife, or scissors, and carefully pierce through the shirt where the holes are.
Take the two strips still attached to the collar and stretch them out as long as you can.
Begin lacing the corset through like you would do shoes, or obviously, a corset.
Try on your shirt and tighten or loosen it as you please!
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