You walk there. Take a look at your Town Map - just head in the general direction of any town/city you haven't visited. Lots of areas that were blocked before you beat the Pokémon League are now passable.
100% of information in signatures on this forum involving percentages is false. If you feel as cheated by this atrocity as I do, don't you dare copy this into your signature.
Hold on... if this percentage was correct at the time of print, that means the actual percentage of false information is less than 100% if this signature is included. Which means that... no! I've become a slave to the system!!