Answer 1

Go to the Giant Chasm through the front entrance. Once inside, go left until you can't anymore. Now, go up. Go trough that doorway.

Ta-Da

Answer 2

I'll do it in steps.

1. Go to Route 22. Go to the cave entrance at the end of the hilly 'maze'.
2. You will be taken to the Giant Chasm. Go downward, to the left, ignoring the first entrance.
3. Keep going left until you hit a wall. Then go up. You will see a cave hole. Follow it, and you're on Route 23.

The path to Victory Road should be fairly obvious from there.