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In order to set up a campsite, you will need to find a flat, open area that is not inside a town, settlement, or hideout. If you try to set a campsite up in an area that doesn't allow it, you will receive a message telling you to find a new spot. You can make a basic campsite without having to purchase anything. You can access the Satchel by pressing SELECT (PS3) or BACK (Xbox 360). These are your tools. Your Basic Campsite will be on the list. You can purchase an Improved Campsite, but you can save with either version. Select the Campsite from your kit to set it up. You can also come across camps that other characters have set up. These appear randomly throughout the game. You cannot save at these camps. When you build your campsite, you will automatically squat down next to it. You can start the save process by pressing (PS3) or Y (Xbox 360). Marston will lie down on the bedroll. When you lie down, time will advance six hours. You can choose to save, or you can cancel to get up. This can be useful for advancing the game time without going through the whole save process. if you choose to save, you will be asked to select a save file. You can overwrite an existing save, or create a new one.
Find an open area. Open your Satchel. Select "Kits". Save your game. Choose to save. Choose a save file.