Look, I have a cushion! Turning my finished tapestry into a cushion turned out to be quite a process so I am really glad to have it finished.
Since this was my first time doing anything to a stitched piece other than framing, I took a lot of steps. I first pinned the tapestry and cushion back together with a ton of pins and then I basted it too with bright purple thread so I’d be able to stitch right between the outer two lines of stitching. I then attempted to machine stitch it as suggested by the instructions and that went surprisingly well for an entire side, As soon as I turned the corner though, my needle snapped, as did the replacement needle. I even moved over to another side of the cushion thinking it might be that specific spot, but nope, the third needle snapped too!
I ended up hand-sewing the other three sides, which was pretty dull. I used backstitch with a double thickness of thread for strength, so it felt like forever, but it was only a few hours really. The cushion didn’t quite fill out the cushion case either, but it’s fine and I’m really happy overall. I’m not sure I want to make another one any time soon though.