You need two leather strips per buttonhole, each strip 4 times wider and about 3 cm longer than the finished welting. With a soft pencil, mark the buttonhole position on the wrong side of the fabric that you have already interfaced with Vilene/Pellon. Transfer the mark to the right side of the fabric with basting threads. Fold the leather strips in half longwise, with the wrong side inwards, and glue together. Pin the leather strips on the right side of fabric so that the raw edges meet over the buttonhole marks. According to the buttonhole length, topstitch in place along the middle of leather strips, securing start and finish. Working from the wrong side, slash between the stitching lines, leaving about ¾-cm gap at each end. Snip into the corners at each end of stitching to form small triangles, without touching the welting strips. Pull the welts inwards through the slit so that the folds meet in the middle of the buttonhole. Fold small triangles at buttonhole ends inwards and stitch along welts from seam end to seam end. Place interfacing over the buttonhole. Mark the buttonhole ends with pins. Cut diagonally into corners of interfacing according to width of welts. Narrowly turn in edges and hand-sew onto welts.