You sort of just do your best and it's Whibley wobbly so it might or might not happen usually is long as you're just in character talking it's going to happen
Yeah, it is more just conversations. Keep in mind public ig posts are that, public. Even plot sees them and read them and can take them into account.
Private messages I have done both conversation style and letter style.
Actions you will want to use IBGAs (in between game action) for. Keep in mind the scope of your action, as some actions arent feasible or might require results of next market day. These are also great tool to clue plot what your interested in and/or what your goals are.
Just to clarify. While plot OOG reads what goes on via the boards no NPC in game has access to that information. If you choose to send a letter or something (a private pm to myself) for an NPC they will respond but otherwise plot will not read what you have written on the board unless it is on a thread that plot starts itself. So if you want to have conversations about how you want to kill the barbarian lord on the boards feel free. Plot will not bring that information into game .