This page of the guide to Monster Hunter Rise provides instructions on how to unlock a sub-camp in the Shrine Ruins.
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 Shrine Ruins, you will find 1 sub-camp. It is located near area 10. Use Wirebugs to run up the wall to higher rock shelves. Then find the vines and climb them to the very top.
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 Izuchi monsters in the Shrine Ruins. Go to the designated region and kill the appropriate monsters. After completing the mission, talk to the merchant again to unlock the sub-camp.