Skyrim Sidelines- Ill Met by Moonlight: The White Stag

Follow your quest marker to the beast- it tends to lurk between Falkreath and Peak's Shade tower, but its movements are pretty unpredictable.  What's more, other than being white it's an ordinary elk stag- this means that you should attempt to slay it with a bow or spell, as it otherwise tends to flee at rather high speeds.  If you aren't skilled with either, this could take some time and/or rather a lot of running around the local terrain, which is more than a little difficulty to navigate swiftly.  Sneaking and archery are very much your friends here, as a skilled ambush archer can easily slay the thing in a single shot by the time they reach Falkreath.

When the white stag is dead, a phantasmal echo of its form will appear over the corpse.  This is an aspect of Hircine, so approach and talk to it.  Hircine greets you neutrally, and then notes the ring you're carrying around.  Get straight to business or praise the Daedra if you like, but either way the subject presently comes around to the removal of Hircine's curse from the ring.  The Daedra is unwilling to remove the ring's curse without some form of payment- which payment comes in the form of a hunt.

Hircine will explain that Sindig has fled and hidden himself.  Despite this, hunters approach the werewolf's hiding place with the intent of slaying him in Hircine's name.  Hircine himself is an amoral being: he encourages this hunt, but only because it is a challenging hunt with a powerful prey.  He doesn't care either way about Sindig, explaining to any accusations of vengeance that he essentially couldn't give a used fig about whether or not Sindig took the ring at this point.  What interests him is the contest between hunter and hunted, and watching it play out.  In fact, Sindig specifically wants you to join the hunt, telling you to go to Sindig's hiding place, slay the lycanthrope, and skin him, offering the hide to Hircine as proof of the hunt.

Hircine encourages you to hurry as well- there is rather a lot of competition for the favor of the Lord of the Hunt.

This grants you a new quest marker on your map, the location for Bloated Man's Grotto.  This is where Sindig hides, and where Hircine's devotees are hunting the lycanthrope.  The grotto can be found to the North of Lake Ilinata, just on the far side of the small mountains there.  There is, fortunately, a narrow pass through the crags that will let you get there without running around the entire swath of rock North of the lake, but it's a touch hard to find- look just a bit to the West of the marker and you should be able to run across it.