Align Your T-Shirts
Lay the t-shirt you want to lengthen on top of the t-shirt you will use as the lengthening fabric, aligning the side seams. Turn the hem of the t-shirt you want to lengthen inside out.
Measure the Extra Length
Measure on the spare t-shirt the amount of extra length you want; for instance, if you want to add 3 inches to your t-shirt, make sure the bottom t-shirt extends 3 inches below the hem of the top t-shirt. Mark and cut off the desired length, measuring upward from the hem of the t-shirt you will cut.
Pin the Two Parts Together
Pin the tube of t-shirt material that you cut from the old t-shirt inside the hem of your original t-shirt. Machine stitch around the hem, placing your stitches over the original hem stitches.
Alternative Way
T-shirts can also be lengthened using ordinary fabric, although the process is a little longer as you will also need to turn a hem on the bottom of the new fabric. Choose a fabric that coordinates with your t-shirt. Measure around the hem of your t-shirt to determine the length of the strip that you need to cut from the fabric, then cut a strip of fabric that measures this length, giving it a depth that corresponds to the extra length you want to add to the t-shirt. Remember to add half an inch to all the measurements to allow for the seams. Sew the strip of fabric inside the hem of the t-shirt, then turn the bottom hem as normal. Coordinate the layered-hem look by adding a little extra length in the same way to the sleeves of the t-shirt.