Okay, so in order to unlock this achievement, you need to find two hidden items on the world map. You should wait until you have access to the Ragnarok before attempting this, since you will need to visit some locations only accessible by airship. However, this side quest can NOT be completed from Disc 4 onwards, so it has to be BEFORE you fly the Ragnarok into the Lunatic Pandora.
Anyway, when you're good and ready, head over to Obel Lake, which is right next to Timber. It is easy to find, as there is a small bit of land that juts out onto the lake. Where this bit of land meets the water, there is going to be a spot where if you press X, it will give you a choice of either throwing a stone or humming. Choose humming and a black shadow will appear. Keep talking to it and it will ask you to go find its friend, Mr. Monkey. Before leaving, take a step back and go back to the coast and choose the rock option until it says: 'The rock skipped many, many times.'
Mr. Monkey can be found in a forest to the West of Dollet. Head into the forest and press X until you find him. You can either sing to him or throw a rock at him. Keep choosing the throw rock option until he throws a rock back at you (you will have to chase him around the forest, though). Go back to the shadow creature, summon it by humming, and talk to it. It should now be giving you hints. Three hints are regarding other rocks, like the one thrown by Mr. Monkey, one is regarding a pillar you can find that ultimately leads to one of the hidden items, and the rest are going to lead you to draw points. You only need to find the three rocks and the pillar.
Anyway, so the four hints are going to be:
'Oh yeah... You'll find something on an island east of Timber, too.'
'Oh yeah... At the beach in Balamb, something special washes ashore at times.'
'Oh yeah... There's also something on top of a mountain with a lake and cavern.'
'Oh yeah, do you know what? Take some time off at Eldbeak Peninsula. I bet it's a wonderful place.'
If the shadow says anything other than one of these, simply take a step back, and reinitiate the conversation until he gives one of these four. The first three of the above are for the rocks, and the bottom one leads to the pillar. Now, each clue corresponds to a location on the world map.
The island east of Timber refers to one of two islands. You will want to land on the smaller island with no beach. Wander around pressing X until you find the rock. The beach in Balamb is easy to find, since there is only one section of Balamb island that has a beach. Anyway, wander around pressing X until you find the rock. Bear in mind, there will be plenty of 'fake' rocks that have no writing on them. It has to be the one that has writing on it. Anyway, the third and final rock location is on top of a mountain with a waterfall coming out of it. If you head west from Dollet and follow the rail tracks you should find it. Wander around pressing X until you get a message saying: 'A bird is warming an egg'. Choose to 'check it out' and you will have to fight two Thrustaevis'. Once the battle is done you will get the rock.
Once you have all four rocks, go back and talk to the shadow. The messages on all four rocks will read:
STSLRM
URHAEO
REAIDR
EASNPD
Of course, FF8 is a Japanese game, and Japanese is read from right to left. Starting in the top-right corner going from top to bottom and then right to left, each column spells out:
MORD
REDP
LAIN
SHAS
TREA
SURE
If you put those together in a single sentence, it spells out the sentence: 'Mordred Plains Has Treasure'. Mordred Plains is a location north of Esthar City that is bounded by mountains to the north and south, and so can only be reached via the Ragnarok. This may take some time, but you will have to wander around pressing X. Now, there are a bunch of different coloured rocks who will give you messages. The only rocks you need to listen to are the red ones, as they will tell you the exact opposite of what you need to do. If they say 'the treasure is west', then head east, and so on. Also, whenever you find a red rock, press X a few times until it says another direction. So, if one says 'the treasure is north' and then says 'the treasure is west', then you should head south-east. You will eventually find a rock that says: 'the treasure isn't here!' Press X again and you will find a Three Stars. However, in order to get the achievement, you will also need to find another hidden item. Remember that other clue about Eldbeak Peninsula?
Head north of Balamb and land on the Trabia continent and head west until you reach the Eldbeak Peninsula. At the very tip of the peninsula press X and you should find a pillar that will say: 'TRETIMEASUREATMINOFFDEISLAND'. Well, remember how the shadow said to take time off? Well, take the words time and off out of that nonsense and you will get the message: 'TreasureAtMindeIsland'. Minde Island is a small island south of Esthar's southwest coast and north of Kashkabald desert. Wander around pressing X and you will eventually find a Luck-J scroll. Once you have both the Three Stars and the Luck-J scroll, you will get the achievement.