I made silk drapes last summer. I was the first time I had wver sewn lined, interlined drapes. You will probably want the interlining, not just for the extra protection from the sun, but for the softer appearance of the folds in the silk. If you go to a home dec or custom curtain shop, the salesperson can show you how it looks.

You can buy the interlining there, too, but it may be cheaper to order online.

Remember to cut all selvedge edges off your fabrics before you begin. No matter how far back from the selvedge you stitch, clip, and press, those seams will draw up shorter than the rest of the fabric, and the drape will hang "funny."

I think with window treatment like these, the rod at the top may extend several inches past the casings and moldings. This allows space for the opened drape to fold gently back and forth (is it called stacking?) and still not cover the window view at all.