Depending on which portal you chose, you’ll access the Ragnarok in different ways:

If you chose the upper left portal, you’ll emerge on the shoreline near the Chocobo Sanctuary. Head inland and enter the camouflaged dome-like Sanctuary, which is north of Grandidi Forest. Acquire a chocobo and ride it south along the eastern coasts of the Esthar continent. Cross the shallow waters at the southern tip towards Kashkabald Desert. Get your Chocobo onto the Centra continent via the southeastern shore and head towards the center of the desert to find the Ragnarok, which appears as a red dot on the world map.

If you chose the middle portal, you’ll emerge on the Centra continent near the Centra Ruins. Head towards a small mountain on the northeastern tip of the island. Go around it to find a Chocobo Forest. Enter and get a Chocobo (or pay the Chocoboy 1,200 gil to find one for you). Mount it and head south towards the beach.
Cross the shallow waters to get to the opposite island (Edea’s Orphanage should be nearby). Once there, head northeast, through a narrow mountain range to reach Kashkabald Desert. The Ragnarok is there and appears as a red dot on the world map.

You’ll notice that with the exception of dungeons and chocobo forests, all other areas are blocked off. There’s not much you can do about the strange barriers surrounding each town, so take a trip down to the Heaven and Hell Islands to stock up on spells. Once you’ve done that, take a look inside the Ragnarok.