You will have needed to complete the the first quest in the series to begin the questline. You can find out guide for that here
You will have to place a total of four signs with ten locations to choose from.
Where And How To Place The Missing Person Signs In Fortnite
This is another quest, like last week with the cat food, where you can achieve the entire quest in one location if it suits you better.
There are five sign locations and Misty Meadows and five at Weeping Woods. You only need to track down four locations and place four signs in total.
The above video comes from Perfect Score on YouTube
All of the signs are attached to structures or buildings. This makes it easier to identify them. You can see their blue silhouette on the buildings which you can approach and interact with to place the signs.
Once completed, you'll receive 30,000 XP and it's onto the next quest for Fortnite Week 4!