Summarize this article:

Cut a 2.5-inch slit in each shoulder of your t-shirt. Lay the shirt flat and cut down from the shoulder along each arm seam to create a 5 inch total cut when unfolded.  Cut away the arm seam if you find that the inside of it shows after making your slit cut. You may also cut a wider V-shape instead of just a slit if you want more skin to show on the shoulder. However, the more fabric you leave, the more gathered and pronounced the final bow shape will be. Cut all the way around the bottom of the t-shirt to create one long strip. Cut this strip into smaller pieces and tie the end of each sleeve to create a bow shape.  Cut two 2.5 inch pieces from your long bottom strip to use as ties. You can make the strips longer if you wish to wrap them around several times to create a thicker middle to your bow. Gather together the fabric at the end of your sleeve below the slit you made. Tie tightly with one of your fabric strips. Make sure you tie the piece in the middle of each sleeve, so it makes an even fanned-out bow shape from the fabric on either side of your shoulder slits. Cut away from the neckline at the back of your shirt, about 4 inches from the top to create a wider U-shape. Then cut out about a 4 by 2-inch strip from the excess material to use as a tie later. The lower you cut the back of the neckline, the lower on your back the bow will be. You can always adjust this cut to be lower later, so cut less to begin. Make a cut all the way across the back of the shirt, 6 inches down from each armpit. Gather the fabric between this cut and the neckline and tie with your fabric strip to create the bow.  Make sure to start and end your back cut just inside the side seams of your shirt. Otherwise, the cut can grow wider around the sides of the shirt.  Tie the bow together securely at the center of the gathered fabric with the scrap that you cut from the neckline piece. Wrap the piece around several times if the fabric allows.

Summary:
Cut a slit in each shoulder for bow sleeves. Cut pieces from the bottom of the shirt for ties. Cut out the neck first for a bow-back shirt. Make a slit across the back and gather towards the top.