Here are the easy steps to this technique. It is the same technique used to bag on a jacket pocket using zigzag basting.
Step 1 – Pin all the intersections that must match. In my Square Dance block I want the block to retain its size and shape even through changes of color (orange to red, for example). Pin in such a way that you can sew over the pins. This will ensure your intersections match.
Step 2 – Using the biggest/longest zigzag your machine can do, zigzag over the join. Catch as little as you can of the folded edge, but make sure to catch it every time. Pull the pins once they clear the back of the presser foot. If you pull the pins prior to stitching you will have intersections that shift and it won’t look as tidy.
Step 3 – Press the zigzag stitching using lots of steam to get a nice crisp fold. This is your guide for your permanent stitching, so do a good pressing job.
Step 4 – Expose the seam on the back side of the zigzagged join. Hand press the seam flat. You will see the threads from the zigzag stitching laying in the valley of the pressed mark. Trim away the excess material beyond the seam. Make sure you look at the enlarged image so you are sure you understand this step. Proceed carefully making sure you don’t accidentally cut into the parts you intend leaving intact.
Step 5 – Stitch the seam using the pressed valley as the stitching path. Pull the zigzag stitching and press the seam again from the front.