This page of the guide to Monster Hunter Rise provides instructions on how to unlock a sub-camp in the Flooded Forest.
Fast travel from the map level to the main camp and sub-camps will make it easier for you to move around the region. In sub-camps, you can also eat meals, change equipment and use the Buddy Board.
In sub-camps, you won't find guild chests with supplies. Large items can only be delivered to the main camp.
In the Flooded Forest, you will find 1 sub-camp. It is located near area 11. Use the vines to climb the mountain.
When you discover the area for a new camp, a notification will appear on the right side of the screen. Go back to the village and talk to Kagero the Merchant. He will ask you to kill 8 Wroggi monsters in the Flooded Forest. Go to the designated region and kill the appropriate monsters. After completing the mission, talk to the merchant again to unlock the sub-camp.