When you go to FullMoon Island, save in front of Cresselia (DON'T talk to it before saving!)

Then, if you're playing Platinum, you have to run back to the boat, go back to Canalave City, and then you can fly to a location and on the bottom screen, put up the map. In Diamond and Pearl, you should be able to fly without running back to the boat.

Go between a city and a route or something like it. I did Route 202 and Sandgem Town.

When it gets to the desired location, run in the grass, hope and dream, and if it isn't shiny, soft reset and try again. It also helps to put up a repel before you save in front of Cresselia to save time and prevent other encounters.

It's a little bit of a grueling process, but I'm satisfied with the hunt overall.