This quest opens up to you after you complete the Of Spars and Sails and ... quest (see related quests). It is given by Barmaid Drinna in Qeynos Harbor at 786, 23, 84 .
You must find some pieces of a map in the Thundering Steppes. They can be found at:
First Piece - Barrel on west coast (near griffons, random spawn location) (1403.90, -1.40, 225.05)
Second Piece - Shrine of the Sirens 1481, -20.4, 239
Third Piece - Pile of Bones in a Ship Wreck 1657.55, -58.51, 105.25
Fourth Piece - The same pirates from the Of Spars and Sails and... quest
After this you must return to Drinna and speak to her. You then must head back to Thundering Steppes to kill Drinna the Pirate at the pirate camp. Killing her results in your reward.