A pretty simple achievement.
Upon hitting level 15 with the Order of Souls, purchase the titles up to (or beyond) that level. Then, you'll need 20,000 more gold. Start up a game as a solo sloop (so select sloop at the menus and choose a closed crew).
When you spawn in at an Outpost, go to the Order of Souls hut, speak with the person inside and open the store. In the top right of the store screen (next to the items you can buy) you should be able to buy an Emissary Flag for 20,000 gold.
Do so, then find the Emissary table (the Order of Souls table is located in the Order of Souls building. Walk into the building and look to your left. Hit [cn_x] to vote on the flag and then this should pop.