How to Make a Sleeveless T Shirt from an Unused T Shirt
To make a sleeveless t-shirt from an unused t-shirt, start by laying the shirt out on a flat surface. Then, use scissors to cut along the seam between each sleeve and the rest of the shirt. If you're going for a casual look, cut the seams off with the sleeves. Alternatively, if you want a neater finish, leave the seams intact when you cut the sleeves off. Finally, once the sleeves are off, try on the shirt to make sure the armholes are even.
Find the right T-shirt. Pull out your favorite T-shirts, and decide which of them you might want to turn sleeveless. Try them on, and see which looks the best on you. Test the look. Roll up the sleeves as high as you can, or tuck them into the shirt around the seam to see if it looks good as a sleeveless T-shirt. Decide how you’re going to cut it. There are two ways to go: leave the seam between the sleeve and the shirt in place, or cut it out. Leaving the seam intact will keep your finished T-shirt from unravelling and looking shabby. It will also make a smaller armhole. For a baggy T-shirt, this is a good approach. Cutting out the seam with the sleeve is a more casual look, and because the hole is larger, a little more comfortable as well. If the armhole is going to be too deep, modify your cut. Instead of following the seam all the way around the sleeve, when you are about 2/3s down the sleeve, angle out into the bottom of the sleeve. When you reach the bottom seam of the sleeve, reverse the angle and cut back in towards the shirt seam, leaving a triangle of shirtsleeve at the bottom of the hole. Trim that to fit. Lay the T-shirt out on a clean, flat surface. If you're cutting out the seam with the sleeve, mark where you are going to cut with chalk. If you're going to keep the seam, poke your scissors into the sleeve about 1/8 inch (3mm) from the seam. Carefully cut around the sleeve. If you're keeping the seam, keep the cut close to it, about 1/8 inch (3mm) all the way around. Be careful not to cut too close to the seam, or it may unravel after a couple washings. If you are cutting out the seam, follow your chalk lines, and cut as smoothly as you can to avoid a jagged look. Repeat on the other sleeve. Keep the sleeves for future projects. When you’re done, you can hem the edges if you’d like, or simply leave them cut. They will curl and soften with use, and help you keep your cool all summer long! Finished.  
Find the right T-shirt. Test the look. Decide how you’re going to cut it. Lay the T-shirt out on a clean, flat surface. Carefully cut around the sleeve. When you’re done, you can hem the edges if you’d like, or simply leave them cut. Finished. 