The Elder Scrolls V: Skyrim/Season Unending

Season Unending involves creating a ceasefire among the two warring factions, allowing the use of Dragonsreach to capture the dragon. This main quest can be bypassed if you instead complete the civil war.

To start the negotiations, you need to do the following:
 * Get support of the Greybeards. This will be impossible if you killed their leader.
 * Speak with General Tulliis. This require a speech skill of 50, which may be boosed as needed.
 * Speak with Ulfric Stormcloak. He should easily be willing to attend the conference.

If any of these aren't possible, you will instead need to complete the civil war quests to have one of the two sides conquer Skyrim.

Negotiations
At the start, Elenwen will be present at the negotiation, causing complaints from the Stormcloaks. Here, you make your first decision, whether she remains or leaves. The decision gives one negotiation point towards the satisified side.

Next, one of the two sides makes a demand for a major hold - either Riften or Markarth. In response, the player can have the other side give up a major hole (Riften or Marakarth), or one of the minor holds. You can only negotiate using terroitories originally owned by a faction - conquered territories remain conquered.

In a short pause, one of the two leaders will likely stand and prepare to leave, claiming that it is unbalanced towards the other faction. After a short speech, he will return to the table, and the Dragonborn can finialize the treaty with other minor terms.

Optimal negotiations
Despite being "balanced", there is an optimal negotiation that gets the most territory.

If you are aiding the Stormcloaks: Have Elewen stay. Make the demands for Markarth in respons to Riften. You should get the option for one additional hold.

If you are aiding the Imperials: Send Elewen away. Make demands for Riften in response to Markarth. Likewise, you should get the option for one additional hold.

For negotiations to work, you need the support of the Greybeards, which is impos and if you previously killed their leader (due to demands from The Blades), then you must complete the Civil War quests for either the Imperials or Stormcloaks.

After the negotiations
Delphine and Esbern will approach, and tell you about the Call Dragon shout. If they haven't already, they will also mention they know about the Greybeards' leader, and demand that you kill him if they are to provide further help.

With the truce active, you can head to Dragonsreach to continue the main quest to capture a dragon.

The civil war quests will remain on hold until you defeat Alduin. After doing so and continuing the civil war, you will need to reconquer any holds given away.