All you do is cripple a flag, just don't grapple it or board it. Once you see the green circles flashing on it, leave and sink some big enemy ships with your broadsides. If you're close to level 15, I'd say use it to get to that level, but I wouldn't rely on this glitch mate. It's more rewarding to level up over time, for me at least. Also, you may want to consider saving for the war galleon first, though I leave that completely up to you. I was able to get my war sloop about five days after I got my war galleon, but part of that was due to leveling my staff on the group of soldiers at Kingshead. Anyway, fair winds and good hunting
