Enter from Virbank City , so you start at the north-east part of the route. Head west from the gate and down the steps taking you south. Once you've moved the last of them and see the Dark Grass nearby, head south-east over to the water. Head onto the water with and Surf south.
When you see the water leading west, follow it over to the waterfall. Use Waterfall and head up it. First, head west. When you reach the stream leading south, head down it. Zigzag between the two rocks and keep heading south until you reach the land at the end.
You'll need to be here during Autumn to access this land. If you're too impatient for Autumn to come naturally, feel free to change the date on your DS or 3DS. Simply set it to March, July or November and re-enter the game. You will need to exit and re-enter the area to force the switch between seasons.
If you need to, head back to where you were and move onto the land, which should now be covered in leaves. There's a Pokemon Ranger here, buried in the leaves. One out-of-place orange leaf denotes her position. Move in front of her and she'll pop out. Challenge her!
Trainer Battle: Pokemon Ranger
Note: After defeating her, she'll give you a Sitrus Berry .
Just east of the Pokemon Ranger is an item, a Sticky Barb . Snatch it up, then Surf west from the bit of land you're on at the moment. You'll reach a cave. This is the Cave of Being . Feel free to enter and explore it, but until you beat the Pokemon League, there's not a whole lot of point.
If you have beaten the Pokemon League, feel free to skip ahead to the Cave of Being part of the Post-Game Walkthrough . Either way, before you go in (or not), there is final item to pick up in this area. Head back to the top of the waterfall.
Don't head back down it, but instead move onto the land just north of it. Jump over the first ledge and turn your Dowsing MCHN on. Use it to find the Big Mushroom . That's really it! Don't forget to set your date back so that it's correct again if you changed it.