Beat Shanks in Alabasta, then beat all the graveyard challenges, then beat Shanks in the graveyard and get the eternal compass, go fight Robin in Sienna Town (the town is on the Inshore Alabasta Kingdom map). When you beat her, she will join your crew and that opens Jaya. Go there and you'll be told to go back to Alabasta and beat Crocodile. Once you beat Crocodile, go back to Jaya and to the waterspout. It'll send you to Skypeia.

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first you gosta get the eternal comp*** from shanks, the one w/no arm. then go kill nico
robin and let er join yer group. then go to jaya. then kill crocodile. then go back to
jaya an inter the wirlpool. bam your in skypia

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all u have to do is go to the graveyards of ships beat shanks and then go left of abalasta
and then u fight nico robin then after u beat her u go to skypia.