You need something plastic so it doesn't scratch the surface, and hard because it's quite stubborn. It also needs to have quite a sharp edge. I ended up sanding a hard piece of plastic that was laying around from those old light boxes from school science experiements. I imagine if you had an old credit card or something similar it would do the job with a bit of sharpening along one edge. The case was actually the hardest part from what I remember.
The tool I was given with the screen which was supposed to be the "official" tool ended up being too weak and bending too easily every time I tried to pry it apart. Despite how easily it seems to work in the video.
If you used the flathead, you'd damage the case. Once inside the case, you need to remove some small, fragile ribbons and also unscrew the metal case holding it all together. Small jewellery screwdrivers are what you need in there. That video I linked to in my last reply pretty much shows you everything you need.