Fiona

TO GAIN THIS FOLLOWER:
Find Fiona @ 53.6, 57.3
She will give you the quest "Swamplighter Queen"
-The Queen is in a cave @ 51.4, 56.5

Next Fiona will give you the quest "Cooking with unstable herbs"
-Talk to Fiona, listen to her instructions, give the dude the elixir

Next, 4 quests will open up. The one you need is "Blademoon Bloom"
-Find Rangari Arepheon @ 55:72
He will give you the quest "Gestating Genesaur"
-Kill the Blooming Genesaur right next to you
When you turn this in NOTHING WILL HAPPEN!

Go back to Fiona @ 53:57 and she will give you a quick turn in quest called "Fiona"
Voila

In order to gain Fiona as a follower, you must pick up the quest Swamplighter Queen and Fiona's Solution from Fiona at (53.6, 57.3). The cave entrance is down in the swamp (lowest level) at (51.44, 57.05), where you'll be picking up the rest of the material.

The next quest from Fiona is Cooking With Unstable Herbs. The existing wowhead comment from Yinxy for that quest says:

Speak to Fiona and wait for her to begin asking for ingredients.

In random order:

Let's add in something dangerous... Maybe even toxic - Swamplighter Venom
OK, how about something that'll stir it up and wriggle around! - Riotvine
Trow in something Meaty - Riverbeast Heart
How about something that'll give it a nice glow! - Moonlit Herb

Administer the potion to Gidwin Goldbraid, then turn in the quest.

Four quests open up. You want the one offered by Rangari Arepheon, Blademoon Bloom. You must go to Blademoon Bloom (55.7, 72.0) and find him huddled behind a tree. Turn the quest in and receive Gestating Genesaur. Turn around, he's pretty much 180 degrees behind you.

With no other quest given, you might think you were lead on a wild goosechase. Return to Fiona and pick up the quest Fiona to receive Follower: Fiona.

Enjoy!

Fiona's Solution is also required along with Swamplighter Queen before you get Cooking with Unstable Herbs