Firstly, you should read our article here to find out what the Halloween event is about if you are unsure about the event.
If you are, then here are some ways which you can take advantage of this event.
Firstly, while the Halloween event is on, you should be walking with your Pokemon using the Buddy feature, as Niantic have quadrupled the amount of candies you earn per km. For example to earn a normal 1km amount of candies, you now only need to walk 0.25 a km to earn that amount, which is great to level up Pokemon.
Also, capturing as much ‘spooky’ Pokemon as possible will be useful for the future, as this new update will take place. So capturing as much Pokemon of the same family, the better.
Walk more to hatch your eggs, as you can earn double candy for egg hatches, so for 5km or even 10km eggs, that could be as much as 40-50 candy per hatch.
This event is for only a limited amount of time only, so try to walk as much as you can as you can earn in some cases 4x the normal amount of rewards!