To use the curtain material you can append the material from the provided blend file, or merge the curtain group directly. The material is built with a base shader where you can plug your own textures. To obtain a good result for the trasnparency effect and avoid repetion, you have to use very detailed and uniform textures like those provided. You can change the overall transparency of the material with the slider of the shader group. You can tweak the curtain traslucency with the slider in the shader group. The shader works well on mesh without thickness. The thickness is simulated by the shader. You can add thickness to the model where you want cloth seams. You have to tweak the texture tiling to adapt to your model mapping. When you need an opaque curtain, you have to use the second diffuse texture or a fabric texture of your choice. To do that, replace the image in the first texture slot of the material with the second texture provided. You can leave the alpha textue as it is, becouse when you reduce the overall transparecy to zero it has no more influence.