Your job is to basically manage everything else the players don’t. If that sounds like a dismissive or flippant answer, it’s not. The planning, documenting, assessment and reactions of every person, group, or collective is in your hands. The first thing to decide in helping you manage this potentially monumental task is two assess 2 starting points: scope and level of detail. Scope will determine how far beyond the direct actions of the player characters you want to be responsible for. The level of detail will determine to what level of granularity each interaction will be managed. After agreeing with the players on these items, your job as the narrator of the action in adventures and campaigns has boundaries that keep the pace and immersive feel manageable for everyone. The greater the amount of information in scope and detail, the more time must be invested. Truly epic campaigns with world politics, large battles, and culture clashes require a great deal more time to familiarize yourself with, track, and change in response to the Player Characters actions in game.
The GM must assess every situation with their knowledge of how the real world works and what players should reasonably expect, what the characters are doing and how the rules express it, what forces are arrayed against them, and what tone are they, as a GM, trying to set. Every situation will be different, and the adjustments you impose on characters will either make the action feel more real or perhaps even slow the game down. Careful not to devolve into rules litigation – this can easily ruin a gaming experience.
Oftentimes a GM must inject an element of randomness into a situation – to simulate elements beyond the absolute control of the direct game elements and to challenge characters. It may be a simple encounter determining reactions of an NPC to a PC (setting the tone for future interactions), it may be that it’s a simple check on the weather, the change in weather, or even if a particular item is stocked by a merchant. There are endless permutations of this, involving many factors in each case; but the judicious use of the Simple Outcome check can be used in almost all cases. Use a skill, attribute or trait as the source of the check is appropriate. Otherwise, assume a character has an average chance of success – level 4 – on any simple check. Any concept relying on “luck” should use the Intuition attribute.
Superior/Full: GOOD – The check favors the character – information or benefit is theirs for the taking.
Simple/Average: INDIFFERENT – The check implies no information or benefit is immediately recognizable; any energy, time or resource spent are wasted.
Failure: BAD – The character is distrusted or rejected.
The particular degree – i.e. an item may be in stock, but it may cost 10x as much as normal, is up to the GM.
However much work you put into a scenario, the PC’s may be outgunned or under-manned. The difficulty in completing any scenario rests mostly with the GM – they have the final say on all aspects of the game. Any play scenario can be adapted on the fly – you can adjust armor, skill levels, abilities, tactics (ambushes, cover, flanking), and even NPC knowledge to handicap or improve the PC’s chances. All of this can be done completely dynamically by a good GM – calculations and changes made as the story unfolds with the PC’s none-the-wiser.
As a general rule, the ability rankings are an excellent way to assess a challenge. The primary ranking of the toughest NPC’s should not exceed the best of the party’s unless you provide a specific means of defeating them or their plans within the story.
There is no uniform way – some GM’s keep all the details in their head to keep things fluid, others won’t deviate from a script. Some GM’s start with a single encounter and make the rest up as they go, others will write everything out ahead of time.
However, things may run faster and seamlessly if you think out place names, NPC names, some basic ideas about them (relative skill compared to the PC’s, gear they have, loot or knowledge they may be protecting) and how they relate to a story thread. Not having to stop to make up these details on the fly significantly helps to keep the immersive experience for the players. Think about the main themes and the tone (scary, dark, etc.), think about how the NPC’s and locale may react to the presence and actions of the PC’s. A smooth story is usually far more enjoyable than one which seems random and has no continuity.