You'll use a fireplace key to open and close the valve. If yours has gone missing, you can buy a replacement like the Stanbroil Universal 3-Inch Antique Copper Gas Valve Key from Amazon for under $10.

Look for a valve near the fireplace. Mine is covered by a round plate with a square hole. Find the key to turn the gas valve. Keys are long metal devices with a square hole on the end, similar to ...