Season Unending

Walkthrough
Should the Dragonborn not have sided with the Imperials or the Stormcloaks in the Civil War yet, Jarl Balgruuf the Greater will demand some form of temporary truce between the warring factions before agreeing to help capture a dragon using his Dragonsreach during The Fallen quest.

The conditions of the truce the Dragonborn negotiates during this quest have (far-reaching) implications for the political structure of Skyrim.

Talk to Arngeir
To get the two factions to talk to each other, it is suggested to get the Greybeards involved, since they are respected by both sides. Their human representative Arngeir at High Hrothgar will not be thrilled, but since their spiritual master Paarthurnax supports the Dragonborn, so will they. After, talk to Arngeir to begin the quest

Talk to General Tullius
Not yet having joined the Imperials, the dialogue will at first steer towards joining them, but persisting to talk to General Tullius will bring up the topic of the peace conference. The quickest way to convince him is to use Persuasion. He will then travel to High Hrothgar.

Talk to Ulfric Stormcloak
Not yet having joined the Stormcloaks either, the dialogue will at first steer towards joining them, but persisting to talk to Ulfric Stormcloak will bring up the topic of the peace conference. Again the quickest way to convince him is to use Persuasion. He will then travel to High Hrothgar.

Talk to Arngeir
Return to High Hrothgar where Arngeir mentions that all is ready for the peace conference. Here the representatives of the Blades, Delphine and Esbern will ask to be part of the conference.

Take your seat
Before the conference begins, the Dragonborn has the opportunity to talk to all the attendees.

The Empire is represented by General Tullius, Jarl Balgruuf the Greater, Jarl Elisif the Fair, Legate Rikke and the first Emissary of the Thalmor Elenwen.

The Stormcloaks are represented by Jarl Ulfric Stormcloak and Galmar Stone-Fist.

The Blades are represented by Delphine and Esbern.

Negotiate a truce
Both sides will present their case, and suggest what they should get out of the temporary truce. It depends on what side the Dragonborn has joined at this point in the game, or stayed neutral.

The goal is to balance the demands of both sides, e.g. a neutral Dragonborn may have let Elenwen stay at the conference table, so that the Stormcloaks demand Markarth (no choice) and this will need to be balanced e.g. against Riften for the Empire, but that will not be enough for the Stormcloaks so they also get Falkreath.

After balancing the demands (or running out of options), the conference will have succeeded, and the Jarl of Whiterun will agree to helping the Dragonborn in the continuation of The Fallen quest.