Mattress pincushions are one of my favorite ways to finish small designs. I've not had a lot of practice and I am not sure where I first learned how to do this type of finishing, but now that I have a few under my belt I think they are fairly easy to finish (just a little tedious to count all that backstitching). I thought I had shared the finishing on one of my designs, but this is the closest I seem to have come:
The technique is similar to simply whipstitching the front and back together like this on a plain cushion.
I've had several questions on how I finished my designs and if there is a tutorial somewhere. I found this link which might be helpful (the blog is in French, but the photos are quite clear and easy to follow--I think it is the tutorial I first used, too--with later modifications--my second and third attempts are much nicer than the first).
Does anyone have a tutorial they can share? I will happily add a link here directing readers to it. Next time I am ready to finish a design into a mattress pincushion (and I am stitching something now with that in mind) I will make sure to take in progress photos!