Sulphur Strips are used when your barrel is not in use to prevent vinegar bacteria from forming.
You want to use one stick per month when the barrel is not in use. Light the stick, suspend it about half way into the barrel using wire or other non-combustible material, and loosely insert the bung. After 5 minutes, remove the sulphur strip (being careful to not burn yourself), and firmly insert the bung.
Thoroughly rinse the barrel before using it again.