A recent request for assistance on the Australian Costumers' Guild forums has prompted me to document a trim I made once for the German housekeeper's costume in the Gordon Frost production of "Sound of Music". It was a common period trim, and turns up on hats and caps too. I made that one from stiff black woven self patterned taffeta ribbon, but it could be made from self fabric if it has enough body. You could seam up a tube of fabric or if it's stiff enough just press the sides under to just overlap in the middle of your finished width. The work is done by hand from the back as the thread carries along.
Contrast thread is used for illustration only, use a best match thread in case the small stitches show through
To work the honeycomb smocking stitch, begin by marking a point in the middle of your width on the wrong side with a coloured pencil a distance in from the raw edge. Next mark your required distance away on one edge and another point opposite it on the other edge, then a mid point at the required distance away, then the edge points and so on along the length required.
Using doubled thread secured in the middle first take a small running stitch out at the edge marking then carry across the back to the other point then along to the mid point, only taking a small running stitch each time. Now pull through and tighten to form folds as in the photo below.
You can manipulate the fullness of the folds, it could be steamed if really neccesary but do not press. It takes a few inches to see the folds forming properly so I did a few samples to test the ribbon and work out the size of the completed trim. I think it was something like 3 times the finished length in the end, it all depends on the fabric. I attached it to the dresses every few inches only, and it survived 8 shows per week plus laundering.
Once you get a rhythm going, it works up quite quickly and looks really great. I made about 10 metres finished all up, in the train going to and from the workroom each day.