Q:

How do I properly complete the quest "In Wolf's Clothing" before doing the "Nameless" main quest?

After investigating the magic explosion with Yennefer, I said I'd join her later on  Hindarsfjall instead of going there with her directly, in order to explore Ard Skellig a little more, playing Gwent with Ermion and Gremist.
When I did get to Hindarsfjall, I followed the upper path to the Missing Persons quest's marker, passing through Lofoten, but along the way, the Strange Beast quest automatically triggered as I passed by its attack site and skipped its preceding dialogue completely. So long story short, I ended up doing some quests before meeting Yennefer.
One of those was "In Wolf's Clothing", and boy was it messy. I triggered some parts of it too early, and the "Talk to the priestesses of Melitele to learn what happened in the garden" milestone, which I couldn't find anything about online, randomly showed up and marked itself as complete. It didn't help that I wasn't on the Nameless quest, so there was no context for the extra footsteps, second cave entrance and the well. All this caused the quest to be even more confusing, and I had to reload to an older save a few times. There are guides out there, yes, but they gloss over certains parts of the quest. So I figured I'd document the full answer I wish I'd gotten, here on Arqade.
(Side note, Yennefer is at the bottom left of Hindarsfjall, near a dock in a bay, which makes me think the game designers initially intended for us to sail to the island from Ard Skellig and meet Yennefer directly, starting "Nameless" right away, but then later added the ability to purchase fast travel maps from merchants to skip boat trips to each island.)

A:

I will avoid being too detailed and will just give some general steps, to not spoil too much. You can make your own inferences and discover the plot on your own.

Pick the quest up from the notice board in Larvik
Talk to Josta in Lofoten
Talk to Einar at Larvik's dock
Go to the front gate of the Freya's Garden, which is locked
Climb the stairs to the left and follow the elevated pathway to a tree. When you interact with a grated gap in the ground, Geralt will comment on how it's too dark to see what's down there. That cave whose entrance icon on the map is in the middle of the garden is part of Nameless main quest
Continue down a tilted pillar to the ground floor with the wolves
Go in a U shape to the other end of the garden
From a mess of footsteps, two paths diverge. The normal one with the blood leads to two locked doors perpendicular to each other, which are also associated with the Nameless quest, although the left one leads to an area you will enter later. The other is Morkvarg's
Follow Morkvarg's footsteps to reach two levers that control the garden's sluices. After the cutscene, continue following the footsteps to a cave (it also has a cave icon on the map)
Go back to the levers and use them to open the rightmost sluice
Dive into the underwater passage behind the small waterfall, to get a Padlock Key
Go back to the tree and unlock the doors of the rooms in the nearby building. Read Morkvarg's journal
Talk to Einar again
Go back to the levers and use them to open the middle sluice, OR jump to the iron gate near the levers and unlock it with your key, OR enter the area through the aforementioned left door
Once in the area, turn right to enter the building with the chain linked to the floor. If you got in through the left door associated with the Nameless quest, you can ignore the well that's part of that quest
Interact with the chain and you will now be able to deal with Morkvarg in two new ways

 By two new ways, I mean either feeding him his own flesh from the chain or giving him the Cursed Fang Pendant.

 Also, if you do lift his curse, you can ask him about his reward before killing him (if you intend to kill him), in order to get his reward on top of Josta's reward.

