To unlock the free Blood Red Riding Hood skin in Modern Warfare 3 and Warzone, you'll need to participate in the Trick 'r Treat event and collect 1,000,000 candies. These candies are earned by playing MW3, Warzone, or Zombies, where they drop from enemies or supply crates. Although reaching the million mark might seem tough, there are several rewards along the way, and you can speed up progress with candy boosts. Zombies mode is the fastest option due to the high enemy count.