once you have caught or defeated ho-oh, you go to new bark and surf west until someone tells you that you have took your first steps into kanto. Then you go through the nearest cave and up the waterfall. From there it's a pretty straight forward path.
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I believe that you can get to Kanto after defeating the Pokemon league and getting the National Pokedex.
yes you can. you can fly back to the indigo plateau and then you can enter the Pokemon leagues again.
beat the Pokemon league and go back to the entrance
In Pokemon Heartgold you need Ho-Oh before going to the Elite Four/Pokemon League so I am sure it's the same for Pokemon Soulsilver.