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 a 2/2 creature -4/-0 so that it's a -2/2 creature, doubling its power and toughness gives it -2/+2, and it becomes a -4/4 creature.

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. The same is true for toughness.