Here’s an obi-style belt that I made to go with a new dress that I’m working on. It uses a nice interfacing technique.
First, cut a stiff interfacing the exact dimensions that you want the belt to be. I used 2 layers of hair canvas glued together with fusible web. It makes a nice stiff surface that’s easy to sew on. Cut the outside fabric (I used necktie silk) slightly larger than your interfacing. Catchstich the seam allowances to the interfacing on the wrong side, as follows:
After the interfacing is catch-stitched, add ties to the ends. Put backing fabric on the wrong side, turn the edges under and invisibly hand-stitch to the seam allowances of the outside fabric.
This is a good way to deal with a stiff interfacing that is too bulky to catch in the seam allowances. It leaves the right side untouched, which is a nice effect when working with fine fabrics. When I finish the dress, I will photograph the belt on it.