Sketch the shape of the welt. Then accurately trace the two halves onto a small piece of the fashion fabric with plenty of room around each, and including the overlap on both sub-sections. You could also just trace the shape in entirety, making it as a one-piece welt. With hindsight that would be a bit easier; but anyway it does look kinda cool as a two-piece welt...
Lay another scrap of fabric under the traced sections and stitch along the top curve, extending the stitching well past the pencilled lines to allow for seam allowances.
C'est fini :)