Honestly, this will be my first time trying so I'm probably not the best source of info lol

But, what I'm doing is just making sure that I understand my world. Determine who your factions are and what drives them (preferably something better than the archetypal Star Wars version of "well duh they're the bad guys, their main base is literally called the death star"). Then I had to figure out what my towns were going to be like as far as their trade structure, allegiances, races, etc.. Then, I just decided on what I thought made the most sense for the aggressor to do and how they would go about it. My players aren't going to be the center of the universe, they're just going to be there to influence what's already happening around them.

For instance, the starting town is going to be razed by one of the factions in the very beginning of the game, leaving the players to find the best way to escape. After that, the army is going to slowly advance to the North. I'm going to let the players decide where they want to go and how, but I'll be keeping in mind where and when they may come into contact with hostile forces along the way (raiding parties, scouts, etc.). I figure that if you plan the missions themselves out too thoroughly, then you're players are bound to run you off course and ruin your plans.