Pulleys are easy enough devices to understand. The pulley is a type of ramp that minimizes rope friction, allowing the rope to transmit energy from a pull in one direction to movement in a different direction. However, with stage curtains, you want them to remain fixed when you aren't holding on to the rope, and to be able to easily open or close them from a single control point. This requires some more detailed rope work.
Make the pulleys -- one for each side of the stage. Cut blocks of solid wood into a disc shape, and use a drill or router to create a hole through the center. Then cut two grooves into the wood, around the circumference of the pulley. Each groove must be wide and deep enough to accommodate the rope, without letting it slip off the pulley when under stress.
Fix the pulleys in place at either end of the curtain guide rod. The pulleys should run parallel to the curtain guide rod. Place the pulley on a post, running the post through the round hole in the center. For larger curtain assemblies, you may want to use a ball bearing mechanism to minimize friction.
Hang the curtains on the curtain guide rod, in the closed position.
Wind the rope, starting from the control point. Wind one end upward over the pulley overhead, then across the stage to the other pulley. Wrap the rope under the pulley and then around the top. Continue to run the line to the other side of the stage. Again, wrap the line under the pulley over the curtain operator's position, and then straight down.
Connect the two ends of the rope together using a water knot. If desired, place another pulley at the operator's position, keeping the rope under tension. Alternatively, provide a post or hitch for the operator to tie the rope to.
Fasten the top of each curtain to the rope, near the point where the curtains meet. The rope should form an "X" at center stage. Fasten one curtain to one rope and the other curtain to the other rope. Do this by sewing a cord to the curtain and then tying it to the rope, securing it with superglue.
Test the pulley system. By pulling on the rope, the operator should be able to get the fasten points for the curtains on the rope to move smoothly in opposite directions, taking the curtains with it.