If an effect instructs you to “double” a creature’s power, that creature gets +X/+0, where X is its power as that effect begins to apply. Toughness is doubled similarly.

If a creature’s power is less than 0 when it’s doubled, instead that creature gets -X/-0, where X is how much less than 0 its power is. For example, if an effect has given Nylea’s Colossus -8/-0 so that it’s a -2/6 creature, doubling its power and toughness gives it -2/+6, and it will become a -4/12 until end of turn.