What the achievement asks
Have all four Stormsong Valley truffle effects on you at the same time: Magic Truffle, Violet Truffle, Valley Truffle and Turbo Truffle. Five points.
Two things make this harder than it sounds. You cannot see a truffle without a Pig Nose, and the four buffs do not stack unless you take them in the right order.
The Pig Nose
Truffles are invisible without it, so this comes first.
The first one is free: it is inside the Carved Wooden Chest in Briarback Kraul, which is also one of the ten treasures of Stormsong Valley. Looting it also unlocks the vendors, so you can buy more afterwards for 25 Briarback Quills each.
The Pig Nose lasts fifteen minutes. That is rarely enough to find four truffles from scratch, so plan on several noses, and farm around fifty quills in Briarback Kraul before you start.
Finding truffles
They grow all over the valley, under trees and in bushes. They are very small, do not appear on the minimap, and are not highlighted. Where the undergrowth is dense you will have to sweep the mouse around to find the cursor change, and first person view helps more than it should.
There are no fixed spots per type: any truffle can be any of the four, so you go round the spawn points until you have what you need.
Three of the four can be bought on the Auction House, which turns an afternoon into a shopping trip. The Turbo Truffle is the exception: it has to be eaten where it grows, so that is the one you go looking for.
The order matters
Three of them keep in your bags. The Turbo Truffle does not, so the sequence is: find a Turbo Truffle, stand next to it, and only then start eating.
Remove every truffle buff you already have, then take them in this order:
Violet, then Turbo, then Valley, then Magic.
Starting with anything other than the Violet Truffle means its buff will not apply, and you will have spent four truffles for nothing.
If a buff refuses to land even in the right order, logging out and back in has been the fix.