This quest is highly randomized and can occur at any time, though the chances of it happening are fairly low. Speak to the townsfolk and if you're lucky, they will tell you of a Spanish flotilla carrying gold and silver from Portobello to a certain town to transport the goods to Spain. Once they tell you of this and your logbook updates, the fleet will begin making its way to the town mentioned so you have to be quick!
The Spanish Golden Fleet always starts at Portobello and ends at a random town (usually Havana). Hence, in order to intercept the Golden Fleet successfully, you should make sure that you're at least close to Portobello before you receive this quest, or you might not make it in time to intercept the convoy.
On the World Map, the Golden Fleet can be visibly seen as a ship with golden sails. Simply intercept it and board them to receive their cargo. Note that they usually have Class 1 or 2 ships in the fleet, but not all the time. The loot for this is negligible to be honest - most of the time you will want to intercept them because they might have a Class 1 ship for you to steal, not because of the cargo they're carrying.
If you have the debugger, you can also "force" the Golden Fleet quest to appear by clicking on F21 and then talking to a townsfolk to "activate" the quest.