Since you've learned how to create and maintain a fire, let's start learning how to harden clay into pottery, which is a fairly simple procedure.
To open a campfire for placing objects "inside" of it, right-click it and and select the open Open option. You can drag items from your inventory into the window that appears, and that is how you cook, smelt, harden, and perform various other tasks. You should now take your clay items and place them inside of the campfire. They will go through various stages of temperature and ultimately harden into pottery items.
The time it takes for the items to harden depends on the quality of the items and the fire, but don't be surprised if it takes around an hour. You will have to maintain your fire and check on it quite often to make sure it doesn't fizzle out, so before you start doing this make sure you have the time to spend on it.