The best way to go about this is to:
1) Read the
2D User Content Guidelines to get a good idea what we're looking for in User Content;
2) Post a preview in this forum (start a thread such as "Jane Vanguard's Flags and Sails") to get community feedback;
3) Once you're happy with your design and confident it meets our guidelines, then you can follow the link from our front page to
submit your flag or sail.
Then it's primarily a matter of time before your flag or sail is either approved or sent back for more work.
Once it's approved you can visit your faction's starter town -- Charlesfort (France), Vera Cruz (Spain), Jenny Bay (Britain), and Marsh Harbour (Pirate) -- and buy your design for a nominal fee.