Standard metal curtain rods and many decorative curtain rods separate in the center, so you can decrease or increase the length to adjust to the window size. You can purchase a straight metal piece, known as a rod extension, that fits into the center of the curtain rod to create an extra long rod. Some rod extensions come with a metal hook support that screws into the wall to keep the extended rod from sagging in the middle. You'll need to purchase a support separately if the extension rod doesn't have a support included.
Pull the two ends of the metal curtain rod apart by grasping the center and pulling each section in opposite directions. The rod should slide apart easily. Use caution--sometimes the metal on the underside of the rod is sharp.
Place the left end of the straight metal curtain rod extension into the left center end of the curtain rod.
Place the right end of the extension into the right center end of the curtain rod, linking the three pieces into one, extra long rod.
Adjust the extra long curtain rod by sliding the extension piece farther into or out of the center ends of the original two curtain rod halves.
Install the curtain rod brackets according to the rod manufacturer's instructions. Follow the directions accompanying the curtain rod extension to install the extension support by securing the hook into the wall. Screw the support hook into a wall stud. You can locate a wall stud by using a stud finder, which is available at your local home improvement or discount store.