You can start the quest by talking to Chet, the NPC located in Crow's Perch
( small fortified city in the middle of Velen). He is just on the right after you entered the city.
(Or the quest might be hanging on the billboard in Crow's Perch, if someone can confirm which is it that'd be great
The quest is advised for level 8 and up but if you are playing on easy you can do it like me at level 5.
(You might want to wait until level 20 if you're playing on Death March difficulty and want the Even odds achievement, even though it won't be an even fight after that
( If you want the Even odds achievement make sure you're not under the influence of a potion from this moment
and take back the mutagen you have equipped in your character sheet
) It has been confirmed that you can keep your mutagen equiped and it will not void the achievement.
This contract will ask you to go to a cave nearby and scare the Shrieker by throwing him a bomb
, (see below to learn how to make a bomb) it will then fly away and you'll have to pursue him in the land until it turns back and attack you. From now on i suggest you to not use any sign, bomb or oil on your sword for the Even odds achievement. (that'll be 1 contract down, and 1 to go)
If you manage to kill the beast and i'm sure you'll be fine don't forget to loot the poor soul and bring the proof back to Chet for your reward and your achievement.
Little tip from Purple DinDono for those brave enough to play in Death March difficulty :
"The defensive option for the fight is to block all the attacks with the sword and ignore the bestiary recommandation to use aard but rather use igni which will hurt him very bad if he's set on fire. " How to make a bomb
(Please NSA don't read any further, it's fine)
To create a bomb you'll need some easy to find herbs you should already have in your inventory and Saltpeter
Saltpeter can be bought from the herborist in White Orchard or from Keira Metz in Velen.
If you don't have the herbs by now, you can buy them from the herborist or try to find 2 Celandine to create the Samum Bomb, they look like that :