Placing warning signs in FortniteTo warn the island of what will happen with this end of the season, you will have to place four warning signs. You can go to three different cities to complete this challenge, but you will probably have to go to at least two, since only Dirty Docks seems to offer 4 (3 for the others).
So we find them in Misty Meadows, one to the north, east side, one to the south and another to the west, all close to the road. You can use the map below.
As for Dirty Docks, we find two to the north, one towards the center and another to the south, towards the containers. You can also use the map below.
Finally, as for Pleasant Park, we find two on either side of the small central square, and another at the entrance to the city to the northeast. Again, they are near a street. You can also use the map below.
To place a warning sign, you just have to interact with it. But you are used to it now.