You can be done now, or if you chose to make the optional cloud curtains, you can now install those.
Thread the twill tape over the black pipe through the gap in the center casing. Using a bodkin or a safety pin, thread each end through the casing on the longer (inside) half of each curtain and out the buttonhole. Loop the tape around the t joint at each crossbar and back into the casing through the buttonhole, and out the other end. Secure to frame with any hitch knot.
The illustration is grayed out for clarity, and the path of the cord holding the curtains up is shown in red.