Insertion stitches are decorative stitches that are used for  joining two pieces of fabric together with  space between them. Most of the time zig zag, looped, knotted stitches are used.
Decorative Laces can be easily and beautifully  joined to fabric this way. (Checkout other methods of lace insertion here).
The insertion stitches gives a look of lace to the fabric pieces – as if you have inserted lace in between.  It was a big part of heirloom stitching. You find  insertion stitches in many of the lingerie of olden times. Kids’ dresses almost always were finished with pretty insertion stitches.
Prepare the fabric for insertion stitching
- Prepare the edges of the fabrics ( serger edge finishing or zig zag stitching would do) and the turn under the finished edges – 1/2 inch. Press in place.
- Take a piece of paper . Any paper would work though simple writing paper has a tendency to tear with sewing ; I use old envelopes which are slightly more thicker and so better . You will be removing this paper after the stitching is over, so it should be easily tearable as well.
- Arrange both the fabric edges so that they are 1/8 inch apart, throughout the length of the fabric (Keep the  space between them 1/8 inch throughout)
Another way is to baste the fabric pieces to the paper on the top and bottom edges only or stitch the whole length, whatever is convenient and easy for you. You can also just pin the fabric in piece on the paper. Using basting stitches ( by hand or by sewing machine) stitch them in place along the edge. If you do not want to stitch or pin use tape to keep the fabric pieces in place. Ensure that the gap is maintained the same consistently. The small gap will widen after you have finished stitching, so ensure that the gap is not big.
Make insertion stitches with a sewing machine
This is ofcourse the easiest way. You need a zig zag sewing machine for this with a decorative stitch function called faggoting stitch and  a zig zag foot. The stitch will look like the below picture  in your machine panelÂ
Keep the fabric pieces together under the needle. You can prepare fabric as described earlier but tacking on top edge would be enough rather than sewing as the paper may prove difficult to remove afterwards.
Align the needle above the space between the fabric pieces.
Start sewing. Ensure that the stitching falls right between the edges, catching the edges with the stitches
Make insertion stitches with hand sewing
Prepare the fabric pieces as above.
Thread your hand sewing needle with more than 3 strands of embroidery thread or single strand of yarn thread.
Start making stitches from one edge to the other
The easiest is ofcourse a zig zag stitch made across the edges. But it is not as pretty as a twisted zig zag stitch. A twisted zig zag stitch is made by lacing the embroidery thread along the fabric edges by looping the thread before making each stitch across.
A twisted zig zag stitch is the most commonly used insertion stitch. You can even do it by inserting beads in between and it looks very pretty and ornamental.
Another very pretty insertion stitch is to use 3 buttonhole stitches ( graded buttonhole stitches or simple buttonhole stitches) along the edges as shown below.
Related posts : Hand embroidery stitches : Decorative Hemstitching
You did not show how to do the stitch!!! No diagrams or anything. I cut a shirt too short and need to reattach it and would like a decorative stitch to do this..
I will post more detailed pictures.