Upon spawning, the Wisp starts to navigate through the map. You will have to keep up with it and make sure you kill zombies around it, as their energy basically 'feeds' the Wisp. If you won't kill zombies around it, the Wisp will slowly decrease in size.
The final destination for the Wisp is the gallows and it'll fill one of the canisters. In case of a small Wisp, a smaller part of the canisters will be filled so make sure it is 'well fed' with energy from the undead!
Once the Wisp arrives at the gallows, Maxis will say that the Wisp's energy is exhausted and you have to manipulate time. With this he means that you have to activate the Timebomb and repeat previous and current step.
After doing so, the canister is fully filled and Maxis will say that the structure has power, meaning that this step is completed.