Faraway Island is a new Island situated OUTSIDE of Hoenn. You need a special ticket to get her

First thing you'll notice is its a labyrinth of sorts with loads of caves and trees making your path difficult
You will find a passage way to go deep into the forest. However as soon as you get there, Mew will spot you and will flee into the grass hiding
You have to follow Mew and corner it basically. Press A when you are facing it and if it is Mew, Bam it'll come out and attack you
Mew is Level 30 so you should bring your Level 30-40ish Pok�mon here, but be warned...if you have any Legendaries, it'll make itself harder to catch with Transform