Step 1: Materials
Curtains: Make them or buy them.
2 Command Jumbo Hooks (~$5 each)
1 medium Command Hook (~$4 for 2)
nylon cord (the more slippery the better)
Step 2: Hang the Hooks
The hooks don't stick well to cold surfaces, so if the wall is cold heat it with a hair dryer before attaching the hook.
If your window are wide, put a smaller hook in the middle to keep the curtains from sagging.
Step 3: Hanging the Rope Rod
Hook the rope around one hook and thread both ends through the curtains.
Bunch up the curtains at one end so you can work with the rope and have a friend hold the curtains up so that they don't pull on the rope.
Tie a loop in one end of the rope. Loop the rope around the other hook and use a trucker's hitch to get the rope taught and tie it off.
Slide the loop so that the knot is on the very end. If it is in the middle, the curtains may have a hard time sliding over it.
Step 4: Enjoy!
(Plus, they will remove cleanly so you don't get fined.)