It really depends on what you want to do with it.
If you want it to catch a spark then most of us aim for a kind of chamois leather like stuff.
Break it out, cut it into flat sheets (well I do, I aim for about 4 to 5mm thick, I don't know how others do it) boil it, let it drain and then gently work at it with your fingers or bash it with a rounded stick on something smooth. A kind of bouncing bash. The aim is to stretch it out fine and thin but keep it whole. Some folks gently work hardwood ashes into it, others soak it in saltpetred water for a wee while. Others just leave it alone.
If I've left it too thick the only way I can stretch it out is to beat it, but the noise irritates one set of neighbours no end, so recently I've just been working at it between my fingers.
Bound to be other folks happy to show and tell
cheers,
M