I had good intentions to produce a tutorial for others wanting to make their own Mini Nesting Mat, but it didn't turn out that great, and there are a lot of things I'd do differently next time, so I'm just going to give a basic overview of my approach.
First, I cut out two big squares from my largest piece, a slightly smaller square from my least favorite color of the four, and four flaps about 3 inches narrower than the largest square.
Then I sandwiched the green square between the two big blue squares, and secured the edges. This piece is to give a little more cushion, and it just needed to be stuck down in a few places.
I put the flaps in place, pinned, and sewed around the edge. Then I used quilt binding to finish the edge.
I will be the first to admit that the quilt binding is a train wreck. I should have finished the edge flat with a zigzag stitch or something before trying to apply it. I also might have rounded the corners, because I didn't do sharp corners very well.
It's possible that Sadie will simply rip it to shreds, but that's ok. I didn't spend very much on it, and I can make it better next time, or just buy one. ;)