If you have a T-shirt that you like everything about except its overly long sleeves, you don't have to give up on it. Even if you don't have experience with sewing by hand or on a sewing machine, it is relatively simple to shorten T-shirt sleeves -- especially if you use iron-on hemming tape.
Size the sleeves to the length you want. Turn the T-shirt inside out and put it on. Looking at yourself in a mirror, experiment with folding the sleeves upward until you find a length that looks good. Fold the fabric outward and toward your shoulder so you can see the excess fabric, which should look like a cuff.
Finalize the fold on one sleeve. Adjust the fold along the circumference of your arm until it is even all around. Use a few straight pins to hold the fold in place.
Determine your cutting point. Take the shirt off, but leave it turned inside out. Lay the pinned sleeve flat on a table. Crease the edges with your hand. Remove the pins, and carefully move the fold outward to extend the length of the sleeve by about 1/2 inch to allow for the hem.
Use the folded sleeve as a "template" to guide you in cutting the other sleeve to size. Fold the shirt in half lengthwise. Lay the folded shirt flat on the table again, with the pinned sleeve on top. Cut the unpinned sleeve underneath, along the folded edge of the other sleeve.
Use the cut sleeve as a template for cutting the other sleeve to match.
Create your folds for hemming by folding up about 1/2 inch of fabric on both sleeves. Make sure the folded edges are even on both sleeves and that the sleeve lengths match. Pin the folds in place, with the pins near the fold to leave room for the hem, which will be close to the cut edge.
For best results, hem with a sewing machine, or by hand with a needle and thread.
If you don't know how to hem by hand or with a sewing machine, use iron-on hemming tape. For best results with a T-shirt, choose a light, thin grade of hemming tape. Specifications, such as iron temperature and settings and how much the tape should overlap the cut edge of the fabric, vary among tape brands. Follow the tape manufacturer's instructions to ensure a quality result.
Iron the newly hemmed edges of both sleeves. Preheat the iron to the highest recommended setting for type of fabric, which for most T-shirts will be either 100 percent cotton or a cotton/synthetic blend, and turn the steam on. Press firmly for a sharply creased edge.