When you reach the BBEG encounter, roll initiative and encounter the BBEG (big bad evil guy).
Try to play the BBEG as true to form as possible, activating attacks and other traits that make sense given the creature’s motives and the circumstances.
Read the BBEG stat block in detail and think about what you think the final boss’s demeanour would be towards the heroes, based on how the story has developed. You could also use oracle rolls to determine whether the BBEG makes certain attacks, changing the likelihood modifier as you see fit.
There is also 52 a Random Monster Combat AI (p.206) that you could use to determine the BBEG’s behaviour.
The BBEG takes their turn after the Event Roll, every round. If your party acquires a sidekick, or you gain a magic item that you can use in combat, perhaps recalculate the boss fight accordingly.
