The player Ship Boards each have three Artillery slots, in which various Artillery cards can be installed. The slots are named Port, Starboard and Bow/Stern, and correspond to Nodes On Deck which must be manned to enable using an Artillery card attack.
To perform an attack, the player must spend an Artillery Order and must have a Crew member manning the Artillery Node corresponding to the slot in which the Artillery upgrade they want to use is installed. There must also be Line of Sight between the player's ship and the target on the Game Board world map. Also, a ship can't use its Artillery if it is on a Storm space.
Note: there are separate Nodes for Bow and Stern on the On Deck area, but only one common Upgrade slot for Bow/Stern combined in the Artillery area of the Ship Board. Some Artillery cards can be used both from the Bow and from the Stern, while others are limited to just Stern (or possibly just Bow).
Artillery can be acquired and installed via the IMPROVE Service when you Visit a Port, or first acquired a Schematic (for example as Spoils) and then installed via the INSTALL Service when you Visit a Port.
Artillery cards are one of the two types of Upgrade cards available in the game. The other type is Improvement cards.
When playing with "optional advanced mechanics", an alternative to using Artillery to attack enemy ships is to instead Board the enemy ship.
Upgrade
Upgrades come in two forms: Improvement cards and Artillery cards. Both types can be bought and installed on your Ship Board via the IMPROVE action when you Visit a Port.
When installing an Upgrade, place it in the appropriate slot on your Ship Board. Upgrades of the Improvement type are installed in slots in the Below Deck, while Upgrades of the Artillery type are installed in one of three special slots marked as being on the Port side, Bow/Stern or Starboard.
If the only slot(s) the Upgrade can fit in are taken, you can discard an older Upgrade to make room for the new one.
Upgrades can sometimes be gained in other ways as well; for example by lucky Fate Die rolls on the Spoils table when Looting a Shipwreck. In these cases, they are usually acquired as Schematics, and need to be installed (usually via the INSTALL Service action when you Visit a Port) before they can be used.