i got confused on that question.....sorry

after you get the eternal post for skypiea you should finish every battles and areas on
your map, including crocodile on alabasta. then you should go to the place on the other
end of the jaya island, the 'east' part of jaya. there you'll get your boat upgraded to
'fly' version. if you already got the 'fly' version, then go to a place below the jaya
island, just press right or R1 till you get it, it's not marking anything, so u just press
X on it. it should get you to Skypiea, the land of angels.

All i did was go back to the area before alabasta and fight robin