It's a tiny bit tricky at first. Lift up the floor panel, then hold it so that the handle grip side is up high and the other end is sloping down towards the ground and angled towards the front of the car. Then you can slide it underneath those big plastic pieces that hold the floor in the higher position, then lower the handle end so that the floor panel sits flat on the spare. If your spare tire is at an odd angle, or over inflated, it will make it difficult to get the floor flat. I had to re-orient my spare a little, but then the floor sits nicely on top of the spare. The only problem that remains is the huge gap between the floor piece and the seats. If you buy a mat and set it on the floor board, then problem solved.
That's why I'm trying to make sure I am buying the correct regular mat for the normal Fiesta that does not block-off the cubby pockets on the sides.