Phione can be summoned by finding 50 scales and going to the well to make the sea bell, if used, Phione will be your pokemon.
Great Guide, but when I tried searching FarFetchd using Ctrl + F it only showed the location and not the rarity. Then I saw there was a typo, its mentioned as "Fartfechd" under Uncommon pokemon.
lol oops, i'll fix it. thanks.
Go to Bluegum Caves (On route 1), on the top right corner of the cave, is the entrance to Bluegum Underground. But you can only access it later in the game after 6 badges etc.