To start with you will need an image with the pattern you want the finished curtains to have. This needs to be at least the same height as  and twice the width of  the eventual image - so if you need to resize, copy, clone, etc., then do that before you start.

We'll be using this attractive floral pattern courtesy of Mayang's Free Textures (http://www.mayang.com/textures/index.htm ):

Now to create the curtains. Create a new RGB image of your desired size with a background color of white.

Select the Gradient tool with foreground/background colors of black/white, choose the linear gradient and set the Mode to Difference:

Working near the bottom edge of the image and using short horizontal (or nearly horizontal) strokes, work from left to right across the image area. The next screenshot shows the position when one gradient stroke has been done and the second is in progress:

Continue to work across the image from left to right. You should build up vaguely fold-like shading patterns:

If you get a criscrossing diagonal pattern like this:

then your stroke has departed too far from the horizontal. Back up a step and try again (this kind of pattern can work well in moderation if you are going for a satin effect but doesn't really work with the curtain texture).

When you get to the far right of the image, repeat the process (still working on the same layer), but this time working from right to left back across the image. Then when you get to the far left of the image you can start moving from left to right again. Basically, keep going until it looks "right". I decided to stop when my image looked like this:

Now since we are not going for a full-on satin effect here, we need to dull down those extremes of highlight and shadow. Add two new layers above this background layer, each filled with 50% gray and set to around 35% opacity. Set the mode of one to Darken and one to Lighten.

Now select a foreground color from the background of your fabric sample. Add a Color layer on top of the layer stack and fill it with this color.

Now to the pattern. Switch to your fabric sample image and resize it with Constrain Proportions unchecked to 50% width and 100% height. This gets you a distorted image like this:

Drag that layer onto the top of your curtain image and set the layer mode to Overlay:

As a final step, add a new Hue/Saturation adjustment layer and reduce Saturation by around 20-30 (whatever seems right).

This created a reasonable approximation of fabric draped as curtains:

A very quick and easy technique and a great spot by Doug! Thanks also to Margaret for her color and overlay insight.