The most common notions of pirate ships are based off the pirate ships that existed during the era of Enlightenment. These ships were often stolen, so there is no unique pirate ship. However, pirates did modify the ships to fit their needs. They would remove projections on the outside to reduce drag. They also removed all excess features to make the ship lighter and faster. For decoration, they would replace the original flag with the infamous skull and crossbones. The most commonly used ship by pirates was the schooner for its speed and maneuverability.
Draw a diagram of what you want your ship to look like. Include dimensions such as the length of the ship, width of the ship, and height of the ship. Also include the height of the masts and the dimensions of the sails. A schooner typically has two sails for maximum speed, although schooners with up to seven sails have been built. The forward mast has a height less than or equal to the height of the mast in the back. The sail in front is typically smaller than the sail in the back as well. You may designate the dimensions of your schooner.
Use the craft knife to cut half-ellipses of balsa wood such that they have the desired height and width of the ship. These are the ribs of your ship, and they will hold the structure of the ship while you build it.
Cut out one thin strip of balsa wood and glue the ribs of the ship to it. This strip will hold the ribs in place so they can support the ship's exterior.
Use the craft knife to cut long strips of balsa wood that are of uniform width. Starting from the bottom of your ship, glue the strips of balsa wood around the ribs of the ship so that they completely cover them. Glue layer upon layer upon each other so that the shape of the hull of the boat is completed. Try to leave no gaps between the strips of wood so that the interior is completely covered.
Cut out thin rods of balsa wood that will form the masts of the ship. Cut out squares of canvas that will be the sails of your ship. Glue the canvas to the masts so that the sails are centred, and glue the bottom of the masts to the frame of the ship so that they hold steady.
Paint your pirate ship however you want. A standard design is a black hull with black sails and the skull and crossbones painted onto the sails.