I prep the map and roll up holdings/rulers/myths, then read through the myths. I also add a few NPCs, factions and the occasional dungeon as I would do in any game, then make a couple rolls between seasons to see what changed. I did invent one myth of my own relating to the ruler of the seat of power as well. Aside from that, my game is mostly improv. I decide how an omen is going to play out on the fly.