How do I play Fortnite Festival Main Stage in local multiplayer?

The launch of Festival Season 7 comes with a new local multiplayer feature on PlayStation and Xbox which allows up to four players to play together on the same console. To use this feature:

• Each local player must be signed in with both an active Epic Games account and a platform account.

• Players cannot play using platform guest accounts.

• All Epic Games accounts must allow T-rated experiences. 

• All of the players need to party up in the Lobby and then enter the Festival Main Stage together.

For more info on local multiplayer, see:  How do I use split-screen mode in Fortnite?