Damage is automatically Max damage for the die type plus roll the die again, exploding (i.e. if the die is d6 and you roll a 6 keep rolling and adding until it doesn’t come up 6). Creatures, including PCs and NPCs, with 1 HD or less don’t get crits: a 20 is just an automatic hit.

Example: a 2 HD wolf attacks and gets a crit. It does 1d6 damage, so the crit damage will be 6+1d6(x). Say it rolls a 6 and then a 3. The total crit damage will be 6+6+3 = 15.