The immobiliser (grey block in the back of the fob) can be a bit tricky prising out with a small screwdriver blade. I got round this by using the old back since it is only the front where the buttons are that needed replacing.
Without the immobiliser the buttons work fine but when you try starting the car a message comes up on the trafficmaster display reading "engine disabled" or something like that
Also the key blade can be difficult to remove. It just pulls out with pliers. Some come out easy, others are a struggle. But it isn't connected to anything, so jiggle and pull like a gud un to get it out. It has it's own bit of black plastic on the end with a rib down one side. There is a corresponding groove in the fob. So it will only go in one way.
The circuit board is held in place by two hook type clips at the wide part. A small screwdriver blade inserted in the gap and twisted will free it easy enough. I struggled to get it back under the clips, gave up and left it a bit loose. Mine still works fine.
To get the back off - with the back of the fob facing you put a screwdriver in the part where the key ring goes and prise it up towards you. Holding it steady use another screwdriver to run along the seam to free the back or it will simply snap back closed. You'll see what I mean when you do it.
Hope this helps