its really easy first, you go through route 129 and u go through route 130 and 131 and ur there
after you beat the elite four and get the mach bike go to the sky pillar just right of pacificlodge town.
you go next into lilycove city.
From lilycove city if you go east you can reach mossdeep city.
go in rustboro city pokemart
Sootopolis city.
Go to Fortree city then go to Lylicove city.you will see it when you go to Lylicove city but you need a Pokeblock case.you will get it in the Pokemon contest in Pokemon emerald.
Sootopolis city but you cannot go in.
sootopolis city
Go east of rustboro city.
mauville city
Go to Mossdeep city and beat the gym.
by using fly or surf