For best effect it needs to be a cardstock with a coloured or white core - something like Core-dinations is ideal. Obviously even with a solid-core cardstock you'll still get the contrast between the distressed finish from the sanding, but it's not as strong an effect.
I use the sanding blocks that I got with my shrink plastic (shrinky dinks), or the file that came in a Basic Grey set. But a fine-grade sandpaper will work fine. You'd probably find it easier to just sand the raised embossing if you wrap it round a small block, I think.
There's a fun technique using sanding and acrylic paints here - Splitcoaststampers - Tutorials
. The block Beate shows is similar to what I use.