Measure and mark two spots along the ceiling, each 3 inches away from the wall. Place one mark as close to the problem wall as possible and the other 12 to 24 inches past the opposite edge of the window. Leave enough space for the curtains to gather next to the window when opened.
Hold the mounting brackets or slide up to the ceiling at each marked point, check for level and mark all mounting screw holes.
Equip an electric drill with a drill bit one size smaller in diameter than the mounting screws and drill pilot holes through each screw hole mark not backed by a framing stud.
Hammer a drywall anchor the same size as the mounting screws into each pilot hole until the head of the anchor is flush with the ceiling.
Hold the brackets up to the ceiling and screw mounting screws through the screw holes and into the drywall anchors. Make sure the brackets are positioned so the curtains slide toward the wall rather than toward the unblocked side of the window.
Attach the top hem of the curtain to the clips dangling from the track. Space the fabric evenly across the length of the track.
Measure the distance of the window from the interior edges of the frame and choose tension rods long enough to span the distance. Make sure the section of frame is at least as deep as the diameter of the ends of the rod.
Screw the small end of the rod clockwise into the larger end until the rod is smaller than the space between the window frame. Thread the rod through the rod pocket at the top of the curtain panels.
Position the rod at the topmost point of the frame and unscrew the small side of the rod until it tightens into place against the frame.
Check for level and adjust the rod as necessary. Spread the curtain panels out as desired.