I posted this on reddit a while back, just adding it here for completion.
When I plan a campaign, I separate the planning phase in two parts:
- Planning the main story arc
- Preparing for a session
Planning the main story arc
First figure out in which world you play want to play. Who are the big players? Kings, Gods, established heroes, etc. .
Create a small list of attributes for every important NPC. The list should contain their motives, morales, personality. The lists will help you to stay consistent and to act upon the PCs actions quickly.
As you won’t be able to tell what will happen in the course of several sessions only make a rough plan of events that will happen.
Set up an initial list of events that will occur if the party of heroes would not exist. Example: the evil guy tries to summon an evil monster in roughly 10 days.
This timetable will give you a rough framework for your campaign, without rail-roading the story. After each session you should revisit the timetable and figure out how the actions of the heroes have changed the course of history. Example: If the PCs killed a commander of the evil forces, the raid on a village in 2 weeks can’t take place.This fixed timetable also means that the players can miss certain events if they are to slow or ignore a story plot. E. g. If the party needs four weeks to stop an event something else will happen without a chance for the PCs to intervene. With the timetable and rough descriptions of the event you got an overall story arc that is not written in stone and can be changed by the players.
Preparing for a session
When planning a session take a look at the timetable to see what is up next. Think through what can happen in the next bit of the story and possible actions of the PCs. You will never find all possible decisions, so don’t even bother writing it down ;).
When you got a rough overview for the session, you can start to prepare.
Create groups of enemies fitting into the scene. Create local maps of villages, dungeons and other sites you want to use. Figure out how the enemies will behave.
Example: 2 groups of goblin guard the mage tower. On a small balcony hovers an all-seeing-eye warning the mages if it sees anything unusual. The door to the mages bedroom is trapped with Trap X. etc. .
If you have difficulties improvising, spend more time describing details of the sites the heroes visit. Have a lot of stuff ready you can use if needed.
At the end you should have a detailed description of the places the party will visit. From that point on the players decide. Listen to their actions and look up what could happen.
Example: They sneak through the fields -> give the goblins and the eye a perception throw and act upon it.
If you notice that it will get to boring, use the assets you created to add more into your world. Maybe 2 goblins decided to plunder the kitchen, and they notice the group.
Hopefully I gave you a rough idea how I plan my campaigns.
- Have a rough story arc for the campaign.
- Create detailed descriptions of sites and enemies for every session and only act upon the actions of the party