In Assassin's Creed Black Flag Resynced, Design Plans are special blueprints that unlock unique cosmetic items for the Jackdaw, such as sails, hull designs, wheels, and figureheads. These plans are obtained by following Treasure Maps and digging up buried chests around the Caribbean. Once a plan has been found, the corresponding cosmetic item can be purchased from a Harbourmaster.
For the Jackdaw's final ship upgrades, you need the Ultimate Plans. There are 12 Ultimate Plans in total. Six are hidden in chests at underwater shipwreck locations, while the other six are obtained by finding Treasure Maps and digging up the corresponding buried treasures. The underwater plans become accessible after completing "Diving for Medicines" main quest, which unlocks the Diving Bell.
All Jackdaw upgrades are purchased from a Harbourmaster at friendly ports. However, the Harbourmaster at Great Inagua is especially important because upgrading his shop unlocks higher tiers of ship upgrades. The first Harbourmaster upgrade becomes available after "This Old Cove" main quest.
To fully upgrade the Jackdaw, you will need Reales, Wood, Metal, Cloth, and the required Ultimate Plans for the final upgrade.
