Take your measurements to determine the size of your tube top. You'll need to measure around your bust, just under your armpits. Make sure to take note of the measurement in inches or centimeters.
Cut a rectangular piece of fabric that is approximately 2-3 times the width of your bust measurement. The length of the fabric should be about 12-15 inches (30-38 cm), depending on how long you want your tube top to be.
Carefully turn the tube right side out through the small opening. Use a blunt object like a chopstick or pencil to push out the corners.