Push one side of the slider window completely closed. Ensure that the pane shuts completely and fits tightly against the edge of the frame. If it doesn't close all the way, check the track and remove any debris that may be keeping it from closing.
Close the other side of the slider using the same procedure used to close the first side. Push the two sides outward toward the outer frame at the same time to ensure they are completely closed.
Push the locking mechanism in the center of each slider down to lock each slider to the main window. If the locking mechanism is hard to move, apply a spray lubricant to loosen it.