Sorry about not posting yesterday, but I was working to finish this and I couldn't stop! I'm happy to report today that I am done with the valance, and it is hanging, and I am very pleased. For once something that I did worked out! It pretty much looks just like I had envisioned. Originally I thought I would do the design across the whole length, but I think this is nicer, actually.
Thanks for all the finishing advice. I wound up lining it with a fairly opaque drapery lining, using the selvage on the lining as the hem, and making it just a smidge shorter than the actual valance, after hemming. Then I added that strip of twill tape about a 1/2" below where the top seam was going to be (I was going to use ribbon, but then saw the twill tape and that worked just as well), stitching it onto the lining first, and leaving a little gap in the stitching every six inches so that I could clip a ring into each little gap. The curtain guy at Mill End Store told me to do this and I think it was a great idea — this way, you dont have to clip those things to your nice valance. Then I just hemmed the sides of the valance, and stitched both pieces, right sides together, at the top. The finished valance has enough heft so as not to get all floppy and awful, so I skipped adding the stabilizer to the top edge. It's hanging just fine, and doesn't have a lot of fullness. The opaqueness of the liner means you don't see the threads carried across the back at all — the individual crosses on the design really pop. I was afraid that if I lined the curtain with something before embroidering it would be impossible to get that lining perfectly smooth, as I moved the hoop along the design? Maybe I'm wrong about that. And thanks to those who recommended not using iron-on interfacing after all — the idea that it doesn't age or wear as well as the rest of the fabrics was something that hadn't occurred to me. So the lining and the actual valance here are not connected anywhere but at that top seam, and I am pleased.
If I had it to do over, I would've spaced my clips so that I didn't have one right in the middle, but I forgot there was a little hook there to hold up the center of the rod.
So, I'm happy with this pretty little dining room (the dark green room is the living room, and it has the same windows, and the bright curtains, and is connected to this one by a big arch). Looking at this photo, I think I should rotate that table 45 degrees, so the chairs aren't parallel to the wall, you know? I'm embroidering those little curtains on the bookcases now (the bookcases are filled with a whole bunch of unattractive junk, so I always have them covered, because one thing our house does not have is storage). I'll show you that coordinating design soon. Right now this sweet little black-and-white needs a walk!