Remove the curtain from the curtain rod. Lay it in front of you on a flat surface, such as a clean floor, with the lining facing up.
Smooth out all the wrinkles from the curtain. Measure the curtain's length or width, depending on which way you want to cut it in half. Note the halfway point with a piece of chalk.
Draw a line with a yardstick and a piece of chalk straight through the curtain's halfway point.
Cut along the chalk line with a pair of extra-sharp shears, making a clean cut. If the shears aren't sufficiently sharp, they won't be able to make a clean cut through the two layers of fabric.