They take away 25%+1 hp from the 'mon that uses the move,
If your pokémon has a total of 100 HP, the doll will have 26 HP.
Here are more info.
And the quote from Bulbapedia:
> Using 25% of its maximum HP, the user creates a substitute with 1 HP more than the amount of HP lost by the user. If the user's maximum HP is 3 or less, it will not lose any HP when the substitute is made. A substitute will have the same current type and current stats as the Pokémon that created it.