We have a king size bed with two box springs. They had to be moved to the other end of the house so I'd have room to do this.
I just placed the panel over the first box spring, and cut it to size. No measuring. I left the lining in place.
And then I got busy stapling with this.
My Arrow ETFX50 T50 Electric Staple Gun. If you don't have an air compressor and you want to upholster something, this gun is a good alternative. You have to use some pressure when you're stapling, but it works. For something like this, though, even hot glue would probably work just fine.
This is how I did the corners. Easy. I was a stapling queen. I breezed through it in about 20 minutes.
And then.....Oh. Crap. Even if I scooted the mattress over more, the staples still showed. (I started pulling them out before I even got a picture. But they showed.) Not really the look I was going for.
So I pulled them all out, and placed the new staples back farther.