Lay the shirt flat. Spread the T-shirt or other short-sleeved shirt out on a flat surface such as a clean table, with the front of the shirt visible. Stand so the neck of the shirt is on your right, and the bottom of the shirt is on your left.
Imagine a line running from the far shoulder to the bottom of the shirt. Picture a line that crosses the T shirt. One end is on the shoulder most distant from you, between the neck and the sleeve, and the line crosses the shirt in a straight line until it hits the bottom.
[1]If you like, you can lay a flat ribbon or other straight, bendable object over the shirt to keep track of where the line is.
Pinch the midpoint of this line with your left hand. Find the point on this imaginary line that divides the shirt into two equal sections: one on your right that includes the neck and sleeves, and one on your left that includes the bottom of the shirt. Use your left hand to reach over to this point and pinch the fabric between your finger and thumb tightly. Make sure you are pinching both the front and the back fabric of the shirt.