1)First you need to do the Walk Through Walls glitch.
2)Go left from fuchsia city towards the cycle route.
3)Go up through the top fence and keep on going up until you see the shore.
4)Walk up onto the water and walk around a bit in any way you want, left right or up (it will automatically make you walk back down).
5)After a while of walking around go back down again and everything should be all glitched up in a glitch city way
6)Then if you want to see what you can walk through and what you cant just surf on the nearest patch of water.
I suggest that the one Pokemon you have with you knows fly and surf (I used my kangaskhan. (yes thats right a kangaskhan with surf and fly)
by the way to get a kangaskhan with surf and fly you have to catch an M or 3TrainerPok?. and teach it Fly then evolve it and teach the kangaskhan surf.