Now weave the same pattern of ribbon in the horizontal direction as shown.
You should end up with what you see below. Push the ribbons together so that there are no gaps showing and pin in place. You will need to do this or the ribbon will move when you iron it.
Cover the ribbon weaving with a cloth and press with the iron. Do not use steam. This will create enough adhesion to allow you to flip the ribbon weave over and then press again to cause a better adhesion and create your fabric. See below. Now, flip the fabric over, and iron from the back. This will ensure that the ribbons stick to the interfacing really well.
Now, take a length of sewing thread with a knot in one side and sew a running stitch along one edge of the ribbon very close to the edge. When you have reached the end pull the thread to gather it to approximately the length of your 4 sides combined. 40 cm in my case. Secure the end of the thread.
Pin to the right side of the edge of the ribbon fabric. Tack the ribbon in place very near the edge. You will not reallt have a seam allowance on the ribbon. No need to have one really as the ribbon has a sewn edge should not frey. There will be a seam allowance on the ribbon fabric though.
Cut a piece of backing fabric that is the same size as the interfacing. I used a little broiderie anglais that I had in my scraps basket. With right sides together and the ribbon in the middle like a sandwich sew around three sided of the pincushion. I did this by hand as it really is too fiddly to even contemplate sewing on the machine.
When you have done this turn the pincushion around to the right side and fill with kapok or any other stuffing that you have. I had some left over from goodness knows when. Sew the last side into place from the back of the cushion and you have done.