1.
Start by surfing at Route 110, south exit of mauville city, to the east.
you will find a "building" there. looks like an underground entrance.
2.
Skip the talk.
Talk once to those guards around Wattson.
After that talk to Wattson and fight your way through the power plant.
you have to find one power switch which is if you stay at the barrier which doesnt disappear on the west.
it is a box.
switch
after that move forward through the power plant until you can talk again to the leaders of magma and aqua.
talk to them, they disappear, talk to wattson.
3.
now find plusle and minun in the power plant.
talk to them as soon as they reach the yellow pillar.
after you did that = both plusle and minun found and disappeared from the map go back to wattson in the power plant.
4.
wattson now tells you to go to the poke centers of hoenn.
basically what you do there is you move up the stairs and talk to the guard there.
the guard will let you pass and you are inside the "teleporter".
talk to the computer at the left inside the teleporter and maybe you get 1 of the 5 fixes or not.
im not sure if this is random poke center or not, for me the pcs from mauville, verdanturf, rustboro, petalburg, oldale, dewford, slateport.
which one exactly? cant remember, just start at mauville and cicle around.
5.
after you did the last of those you have to fight william or however he is called.
he is at one of those "machines" inside the teleporter room.
you cant overlock him^^ :)
he will use some strong pokes including a mega gallade.
tbh. i had a dragonite, typhlosion and gardevoir with me and i defeated him(all lvl 100).
btw. you can leave this teleporter and get to the pc to grab some lvl 100 pokes as well ;).
over all: easy fight.
6.
after defeating this william or whatever he is called you get teleported to our old friend wattson at his gym.
blablabla.
you can use the teleporter.
hope that helps a bit