This is a highly missable trophy in Chapter 4. If you really pay attention and read and talk to everyone throughout this Chapter you can piece together how to turn someone into a Werewolf, but the game doesn’t outright tell you how until Chapter 5 when it’s too late so you’d have to do it in another playthrough.
To turn someone into a werewolf you need 2 specific things. You need a silver knife or machete and you need wolfsbane.
You can find some silver knives in a secret underground area of Absolution Hall which is where you start the Werewolf journey. If you interact with the drapes on the wall where Cleric Franciscus is, you’ll find a switch behind them which will move a statue allowing you to go down. Down here you can find some silver knives, as well you’ll be able to get another trophy here later (Power Behind the Pulpit).
One of the main quests will task you with finding and collecting Wolfsbane and you’ll be able to gather more than you need for the quest. Collect as many as you can and you’ll be ready for this trophy.
While you can technically turn ANY posse member into a werewolf, the trophy only counts for a Bounty Hunter. The hero from Journey 1 counts as a bounty hunter so you can recruit her from her home and turn her into a werewolf if you’d like.
I chose instead to find a generic person out in the world to turn. Now the only real way to know WHO is a Bounty Hunter is by checking their name which will say what they are. Unfortunately there doesn’t seem to be any correlation between their pictures and their role, so you’ll need to travel to different towns with people for hire and find anyone that has Bounty Hunter in their name. Things like Adept don’t matter and it will still count just fine. Hire them into your posse and we’re ready for the next step.
Take one of the Wolfsbane and put it in the Bounty Hunter’s inventory. Also make sure that you have your silver knife equipped. Next we need to find wild wolves. The easiest way I found to do this was to camp near the woods and I almost always found wolves.
Once you're there and you’ve confirmed there are wolves, make sure it’s night time, you can loiter if you need to. Save your game and we’re ready to go.
You need to kill one of the wolves with the knife, using melee. If you’re worried about your posse stealing the kill, you can talk to them and tell them to go unarmed so they can only melee as well.
Approach a wolf, kill it with the knife and the ritual will be complete and you’ll get the trophy.
Here is my video guide with all the details: