For t-shirt yarn, you need t-shirts with no side seams, preferably unused.
You can cut a seamless loops by cutting across the cross-section or you can get up to 200 feet (from an extra extra large t-shirt) of continuous yarn using the spiral cut technique.
What Type of T-Shirts to UseEditUse only new unused 100% cotton shirts that have no side seams. The lightest weight cotton undershirt that you can find is best. Heavy t-shirt material does not work well.
In the U.S. Hanes and Fruit-of-the-Loom lighweight undershirts are preferred sources.
Strips from 1/2" (1.25 cm.) to 1" (2.54 cm.) wide seem to work best. Some people report that 1/2" to 3/4" strips work better than wider strips. In August 2013, Edward compared 30" top-string loops made from 1/2" and 1" strips and found that the 1/2" strip outperformed the 1" strip. Some people, however, cut strips up to 1-3/4" wide.
The strip width refers to the width at cutting. The strips become less wide upon stretching (see Preparation).
Some people have been known to make twisted cord from 1/2" strips of t-shirt yarn.
It is recommended that you wash the t-shirts in the washing machine before using the material. T-shirts are treated with sizing that inhibits bubble formation and which washes out with laudry detergent. We recommend that you wash the t-shirts in hot water and machine dry for maximum shrinkage.
After cutting the material, pull on it to stretch it as much as you can (without tearing it). The material will curl on itself which is beneficial. Even if machine washed and dried, the material may shrink after it has been used and dried though it does stretch a bit when wet.
T-Shirt yarn typically doubles in length when stretched.
People use strips anywhere from 1/2" (1.25 cm) to 2" (5.1 cm) wide. Some people have been known to make twisted cord from 1/2" strips of t-shirt yarn.
For even strips, use a Rotary Cutter, straight edge (ruler) and cutting mat. These can be found at fabric and craft stores.
The circumference of the body of a Fruit-of-the-Loom 3XL men's undershirt is 50" unstretched. The resulting strip when stretched is about 90-98". Cut a 1/2" to 1" wide strip for a great 30" or so top-string loop. [NOTE: Aug. 30, 2013: this t-shirt had been machine washed and dried prior to cutting. After the first use, the loops shrunk from 98" circumference to 64" circumference! I plan to boil the next batch of shirts to see if that influences things. I was able to create tubes well over 30 feet long with the 1/2" width strip -- which actually seemed to work better than the 1" strip -- but that was just one session.]