Hold the ladder with both hands so the base and fly section of the ladder are vertical.
Use one hand to grab the rope hanging down the center of the ladder, usually behind the rungs.
Pull on the rope. Note how the fly section of the ladder extends. Hold firmly onto the base section as the fly section continues to extend. Watch carefully for how high the ladder is becoming.
Let go of the rope when the extension ladder is slightly higher than your desired height. The fly section will drop slightly until the rung lock catches a rung, securing the fly section into place.