Purchase a telescoping drapery rod kit and follow the manufacturer instructions.
Climb a ladder and measure 1 to 2 inches outside your window frame and 4 inches above it. Mark with a pencil where you need to install the rod bracket screw holes.
Drill the holes into studs or use drywall screw holders to ensure your rod will be strong enough to support your drapes. Tightly screw in the screws that will hold your brackets with a Phillips-head screwdriver.
Screw the support brackets into the wall on one side. Place the telescoping curtain rod closest to the wall into the bracket. Use a carpenter's level on top of the curtain rod to be sure the rod is level before screwing in the second end.
Have someone observe whether the bubble on the level is between the center lines. Move the rod slowly and adjust the rod until the level is balanced. Repeat with the second curtain rod. Screw the second bracket into the wall on the other side.