Becoming a Pirate
First thing first, you need to be in a guild. To do this, you just need a house and a guildstone. Once in a guild, its time to head to Buccaneer's Den. Remember that you cannot Recall or Gate to/from Buc's Den so you need to head there by boat. This is the next step, obtaining a boat with cannons!
Building a Ship
You first need to see a shipwright (suggestion is the Oaken Oar just south of West Brit Bank) and purchase a boat plans for whichever boat you wish to craft. This is also where you buy a guild dock deed. To fill the boards requirement, simply drop the amount of boards you need directly onto the plans, and it will add it. Single click the plans to see how many boards have been filled. It shows up like this (4539/5500).
Once you have filled the deeds boards requirement, you open up the Carpentry gump and select Boats and Cannons and the correct boat. One of two things will now happen. You may fail and some of the boards are lost in the attempt. You would then need to add more boards to the deed and try again. If you succeed, your new ship deed will appear in your backpack, ready to use.
REMEMBER, ship deeds (like house deeds) can be stolen, along with the keys to the ship. So be very careful where you do this work.
Controlling the Dock
You have to take control of the docks at Buc's Den in order to collect doubloons, which is pirate currency. To do this, pull your boat up alongside the dock with the cannon facing the dock. You need explosion postions or kegs in order to charge up the cannons. Once you have added explosion potions to the cannon, simply double click the cannon and target the Cannoneer's large cannon. If you happen to get your hands on gun powder, the damage output seems to be less than explosion potions but you can fire the cannons more frequently. There are several cannonners on the docks and one ship will find it very difficult to survive the attack.
You can also attack the cannoneer himself, but they take no damage and it will take many more hits for the cannon shot aoe to destroy the cannon itself. So target the cannon and it will be destroyed in 2-4 hits depending on the cannon size and ammo used. Notice you will take damage during this time so make sure to heal yourself.
Your boat also takes considerable damage throughout this event, but should not be enough to destroy it unless its durability is very low to begin with.
Now that you have killed the cannoneer, you need to remain at the dock. For every 10 minutes you are in control of the docks you get 10 doubloon added to your boats hatch. You must remain in your boat and at the docks or the cannoneer will reset and you have to kill him again, losing control of the docks until you do.
You can spend your doubloon at the NPC Pirate Vendor wandering the woods south of the Pirate's Plunder Tavern. Your guild must be in control of the tavern to purchase goods from the vendor.
Entering the tavern will give your guild control of the tavern, laying down a large pirate symbol on the floor, as long as your guild members are the only players in the tavern. If there is another guild there, you must wait for them to leave or kill them to take control.
You can single click the boat tiller to see the ships durability. The lower the durability the slower your boat will move. While under attack, your ship will eventually catch fire. Once this occurs the boat will no longer move. The fire itself does not cause continuing damage, and will eventually go out on its own. This will not improve durability; the ship still needs to be repaired. The only way to repair boat damage is to dock it at your guild dock. You can dock you boat by saying "dock" and paying the dockman 25 gold. To retrieve your boat the command is claim.