How do you Get to Johto region at Kanto region?

None, Mew is from Hoenn

compllete your nat dex

You can`t it`s imposible exept in Emerald. I've heard that in Emerald after you get the diploma from the game designer in the Lilycove Motel you go to a PC in any center and a Johto Pass will be...

HOPE THIS HELPS

it's both but you start out in the johto region then in some point in the game you get to go over there