It has the added benefit of kind-of-obscuring the view for nosey neigbors while still letting the light in. (I left the blinds in because my neighbors are extra-nosey.)
Reasonably sturdy hardware cloth about one third to half as wide as your window is high--I used two-foot wide quarter inch mesh
Four sturdy hooks, eyes, or some other relatively heavy duty fasteners
EITHER one additional fastener per foot of window OR a staple gun and staples
Enough S-hooks to hold the stuff you need held
Step 1: Measure and cut
You shouldn't need to measure the height, as the rack should only extend about halfway down, leaving you room to open and close the window--and for cleaning, if you're into that sort of thing.