To start this sidequest, you have to go to Froki's Shack . Where is it? There are some ruins south of Ivarstead . Its southeast of these Ruins. Speak to Froki and ask him about the Sacred Trials and the Quest starts.
We need to find and defeat the Guardian Wolf, Skeever and Mudcrab . Travel to these three locations and you'll meet with these guardians. These appear as Ghostly Entities , and are spread out over the map so ideally you're doing this quest later in the game so you can fast-travel closer to each location. These are the easiest enemies of the game (well, maybe not the easiest, but you shouldn't have a problem).
Each spirit will have multiple regular animals of the same type to back it up as well, so be ready to slay a pack of beasts each time you head out. Return to Froki once you've defeated them and now he tells you where the Guardian Bear, Mammoth and Sabrecat are. Yep, we have to go kill some more ghostly animals.
These are stronger enemies, obviously, but still rather easy. The Sabrecat will have some wolves to back it up, but the other two beasts will be alone. Return to Froki and now you need to take care of the Troll champion . Go to Graywinter Watch (by the " Ritual Stone " standing stone, check the map).
There are two normal trolls in here, as well as the champion. Take care of them (sneak attacks help a ton) and loot the place. You'll find the ARCHERY SKILLBOOK "Vernaccus and Bourlor" on a barrel in here as well as Froki's Bow .
Return to Froki and you'll obtain the item ' Kyne's Token ', which reduces damage from animals by 10% and makes bows do 5% more damage. The quest will now be complete as well. Congrats!