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If you don't already know how short you want to cut your shirt, put it on, and make a mark on it where you want the new hem to be. You can use a dressmaker's chalk, dressmaker's pen, or even a sewing pin for this. Take the shirt off when you are done, and keep it turned inside out. This may also work on other shirts made from stretchy material. It is not recommended for shirts made out of woven material, such as linen, due to fraying. If you want to be extra neat, draw a line using a dressmaker's chalk or pen first, so you will know where to cut. Use a ruler to measure up from the bottom edge of the shirt as you draw the line. This will ensure that the shirt will be the same length all around. This will be the inside of your hem. Your shirt should now be the exact length you want it to be. You don't have to double-fold or finish the raw edge, because T-shirt material does not fray much, if at all. If you want a nicer finish on the inside, then you can serge the raw edge, but it is not necessary. Make sure that you use a heat setting that is safe for the material your shirt is made out of. This will give your shirt a nice, crisp edge all along the bottom. If you do not own a sewing machine, or if you do not know how to sew, slip some iron-on hem tape into the hem first. The results will be stiffer, but at least you won't have to do any sewing. Use a color that matches your shirt, and remove the pins as you sew. For a more professional finish, you can use a double-pointed needle. Alternatively, you can sew a second line right beneath the first one; this only works with the stretch-stitch, however.  If you are using iron-on hem tape, simply iron over the hem following the instructions on the package. Be sure to sew back and forth over the start and end of your sewing a few times to prevent unraveling. Try to start sewing at one of the side seams. This will help conceal the start and end of your sewing better. Your shirt is now ready to wear!
Turn your shirt inside out. Cut your shirt ½ inch (1.27 centimeters) longer than you want it to be. Fold the hem up by ½ inch (1.27 centimeters). Press the hem flat with an iron. Secure the hem with sewing pins. Sew as close to the raw edge as you can using a zigzag stitch or a stretch stitch. Snip off any excess or loose threads.