So the first step is to figure out whether you want it till your navel or just below the belly button.
Once you have done that, cut the piece into two halves
Now you need to sew the edges using a needle and a thread. Make sure to leave a small loop or hole on one side
Take the other half of the fabric, measure 2" (inches) and cut horizontally. You will have a long round loop. Cut one of the edge so you have a straight line. Tie to a safety pin and insert through one loop of your cropped top and take it out from the other end.
I've changed the neckline to V-shape and cut the sleeves as well.