Skaven Wizards begin the game with a number of Warpstone tokens equal to their level.
Before casting a spell, a Skaven Wizard can declare that he is eating some of his Warpstone tokens to boost his powers, generating extra Power dice. He can eat any number of tokens to cast a spell, as long as he has enough tokens left. When casting the spell, roll one extra dice for each Warpstone token used. These extra dice are added to the casting total, counting as normal Power dice and can cause a Miscast and Irresistible Force.
This allows the Skaven Wizards to use more dice than they are normally allowed for their level. You can cast a spell without using any normal Power dice and only with warpstone generated dice if you want.
For any of the warpstone generated dice that rolls a 1, the Skaven Wizard suffers a wound, with no saves of any kind allowed. The results of Miscasts still apply as normal to Skaven Wizards.
When using both normal Power dice and warpstone generated ones, it is best to use dice of a different colour to represent the warpstone generated Power dice (green is best!), or to roll them separately.