Hello all!
With the release of Patch 2.8 comes a brand-new prologue—and a new way of determining your Wayfarer's mentor. The following is a walkthrough on how to get a specific mentor. I would still recommend playing the new Prologue blind and seeing what happens first!
This guide contains spoilers for the new prologue.
Choice One:
The two skill checks during the fannarl fight are counted together. Passing/failing doesn't matter.
- Strength or Agility Only: +2 to Cenric
- Perception or Persuasion Only: +2 to Varyn
- A combination of Strength/Agility/Perception/Persuasion: +2 to Sero.
Choice Two:
The Timed Choice.
- Attack: +1 to Cenric.
- Escaping through the window/Looking inside the crates/Make a distraction/Run and escape up the ladder: +1 to Varyn
- [Abstain]/let the timer run out: +1 to Sero
Choice Three:
If you choose to interact with Sabien
- "Yes. It is": +1 to Cenric
- "No. It's not.": +1 to Varyn
- "Why does it matter? I'm here now.": +1 to Sero
If you choose NOT to interact with Sabien ("Shake your head. You don’t want him coming anywhere near you.")
- +1 to Cenric, Varyn and Sero
Choice Four:
Who you ride with.
- Ride with Cenric: +1 to Cenric
- Ride with Varyn: +1 to Varyn
- Ride with Sero: +1 to Sero
If two or more masters have the same amount of points, mentor choice will be randomized. So, for example: if Cenric and Varyn both have 3 points and Sero has 1 point, you have a 50/50 chance of getting Cenric or Varyn (but not Sero). If Cenric, Varyn and Sero are all tied for points, there is an equal chance of randomizing one of the three.
Guaranteed Cenric 
- Strength or Agility options ONLY
- Timed Choice: Attack
- Choose to interact with Sabien and select "Yes, it is."
- Ride with Cenric
Guaranteed Varyn 
- Persuasion or Perception options ONLY
- Timed Choice: Escaping through the window/Looking inside the crates/Make a distraction/Run and escape up the ladder
- Choose to interact with Sabien and select "No. It's not."
- Ride with Varyn
Guaranteed Sero 
- One Strength/Agility and one Persuasion/Perception
- Timed Choice: Abstain/let the timer run out
- Choose to interact with Sabien and select "Why does it matter? I'm here now."
- Ride with Sero
Have fun!