The main method a GM should use to accomplish this is to invoke the past using the techniques of Call-Backs, Recurring NPCs, and Pre-Player Setting History. A game-master adds in references to the history of the setting or important historical events including ruins, stories and legends, through the mouths of NPCs, and via artifacts, relics, and place-names.

This pre-player history is the built-in history of the setting and game world which exists independent of the PCs because it predates them. It often serves to explain why the world is set up as it is when the PCs start. It serves to allow the GM to arrange certain scenarios especially when it comes to culture clash and social-politics without the hassle of having to invent and work out the explanations themselves on-the-fly. Whereas pre-player history cannot normally be influenced by the players, call-backs to what they have done or seen allow them to take a hand in building their own history.

Call-backs to previous events which can be from previous sessions or the results of a historical event help to keep the past cycling into the game. A call-back is a simple reference; again the methods used can be utterances from an NPC or visible signals hinting as to what the cause of the current situation may have been. These are especially useful when referencing something the PCs have done or when they’re facing the consequences of their previous actions. This brings us to the recurrence of NPCs.

Recurring NPCs are another tried-and-true technique to invoke the past especially when it comes to villains that have either gotten away or cheated death in some way where the GM can either choose to just have the NPC announce themselves in-game or leave some clues for the players to follow in order to discover who’s really behind certain current events. Note that recurring NPCs should be memorable characters in the first place as if they simply blended into the background it can be a little underwhelming when they show back up, if the players remember them at all that is.

A clue to what the players see as a worthwhile NPC are the ones they continue to talk about and those which they suspect repeatedly. The GM needs to pay attention to what the players are paying attention to and what they remember about not only the previous session but which NPCs, items, and places they can recall easily as well.

The GM should know what tabletop tales the players like to recount and tell. This not only helps with building mythology but also arms the GM allowing them to mess with players by conflating or deflating by rumor milling and the retelling by NPC’s or an enemy poet/bard of the PC’s exploits. Which in turn can deepen a mythology and set-up mythical cycles written from inside the game by NPCs.

Songs and poems can be altered by bad memories or artistic license when being repeated by NPCs. These can mutate and grow wildly in the telling, retelling, and the copying when scholars and poets use the works of others to build upon or cherry-pick scenes and characters which serve their version of the story better. Names and identities can be altered, eliminated, merged or wholly fictionalized in the telling. Especially over an extended amount of time with only the more popular versions surviving obfuscating any facts that may have entered into the original work(s) when building off of a pre-existing mythology. It is also not uncommon to find differing popular versions in various places limiting certain versions to specific regions or traditions.

This invoking of the past helps to build a coherent body of material that is more easily ordered and thusly lends itself to easier narration. Basically it just makes things easier when it comes to creating a tabletop mythology as well as adding depth to the game allowing for a melding of history and the stories of the player-group which can be used for another campaign which uses the previous as its underlying mythology and history.

Even so, to really analyze the campaign its body needs to be dissected arming the potential mythographer with knowledge of its structure so the campaign material can be broken into the medium with which the mythographer composes myths. In order to do this effectively it helps to know the shape that TRPG narratives take and the structure of a TRPG campaign itself.