PF2 - How do I activate flanking?
 

Hi.
I'm overlooking this I guess.
What steps do I do to set my flanking bonus?

In PF2 flanking isn't a bonus. It gives the flanked person the flat-footed condition. Also note the rule about being flat footed until you act in initiative does not apply to most characters. The only class that gets that ability is a rogue.

Yep, flanking is now a GM adjustment.
Being flanked is now a player adjustment.

Thanks for clarifying this for me.
So, if I flank somebody, I have to make sure the GM knows about it.

If I become flanked, then I am subject to the "Flat-footed" condition, pg 322. To set this in HOL, I go to the Effects window, select "Conditions" and add the appropriate condition. This adds the selected entry to your effects listings window, and you can toggle it on/off as needed in the future.