Start with a new image with the dimensions that you require. Set the background to white. Select the paintbrush tool. Set the materials palette to gradient, with an angle of 90 degrees and the repeat set to a reasonable number
Use a largish paint brush with the blend mode set to difference. Paint vertical lines fairly widely spaced. Make sure that the lines extend from top to bottom.
Now fill in the gaps between the lines you have just drawn by painting more vertical lines. Now, depending on how tightly you want your folds to be, paint down the dark lines on your image. Because you are painting in difference mode, these will be lightened. (If you're unhappy with some of your folds, erase them with an eraser brush large enough to remove them, and then repaint with the gradient brush until you get what you want.) This is what my final curtain mask looked like:
I didn't go to as much trouble as Leah in finishing off my curtain, I created a new layer and filled it with one of PSP8's default patterns. What worked quite well for me was to put the pattern layer below the curtain mask layer; set the curtain mask layer to overlay; and then reduced the opacity of the pattern layer to 70%.
So it can be done in PSP8 as well!!
Thanks to Doug, Margaret and of course Leah, for documenting this technique for PS.