Welcome to inside Ness's mind! Feel free to wander around and talk to people. Note how the background changes color every time you speak to someone! Unfortunately, once you leave Magicant you can never go back, so have as much fun here while you can.
You can find Tracy and your mom over to the top right of the starting area. Your mom lets you rest and Tracy still manages your items. If you have the Sea Pendant on Ness, equip it. Otherwise you'll want to buy an Earth Pendant from the Magicant shop to protect against fire, freeze, and flash. Also, buy some Magic Pudding if you don't have any PP replenishing items. You'll need them.
After walking around a bit, you'll notice that there is a home with "Flying Men" inside. You can have a single Flying Man join you if you wish. It is recommended that you do so, as you currently do not have any other party members with you. Unfortunately, the Flying Man will not last terribly long if you keep fighting. You can always come back and enlist another, though.
Check you level. If you are at level 90 or higher, run from every battle. There will be a rather massive amount of leveling at the end of this area, but it won't bring you higher than level 99. In other words, if you are at a high enough level, conserve your items and strength for the upcoming boss battle. If you are under level 90, you can fight but you don't necessarily have to.
Once you are ready head to the right outside of the Flying Man shack and into an area populated with enemies. These enemies are rather difficult, so it is recommended you do not spend too much time fighting them. If you must, head back to your mom to rest up. If you encounter any Care-Free Bombs, do not spend the time to fight them. They deal a great amount of damage and take a few rounds to defeat. Run away.
At the first fork in the red path, head to the right for a Bag of Dragonite and a Baseball Cap. Head back. At the second fork in the road, take the upper path for a Goddess Band. Now just follow the path around picking up a very useful Magicant Bat and a Magic Tart on the way. Once you reach the odd tentacle, use it to teleport to the Sea of Eden.
 The Sea of Eden
Within the sea of Eden you will encounter only three Krakens. These Krakens can do fire damage, so make sure you have something that protects against it equipped. They shouldn't be too difficult to defeat.
At the end of the Sea of Eden (it is not very large) you'll find that same Mani Mani statue that you have encountered throughout the game. Walk up to it to begin battle with Ness's Nightmare.
After the battle you'll be told by yourself to head back to Saturn Valley, but before you can go anywhere you will receive a massive amount of levels and stat boosts. You will also learn any PSI powers you haven't gotten yet. You'll wake up in Fire Spring and the sound stone will be gone. Once your party is back in order, teleport to Saturn Valley.