It's not totally clear how to start Sunbreak, and with a game like Monster Hunter Rise, that requires hours and hours of hunts in order to progress, it makes sense you'd want to know what you have to do in advance. No one wants to buy an expansion to a game and then find that they can't actually play it on release.
Even though it's time-consuming, the way you actually unlock the expansion is relatively straightforward. If you're planning on playing it straight away, you might also want to know the exact Sunbreak release time. That said, here's how to start Monster Hunter Rise: Sunbreak.
It's worth noting that a lot of players are also having difficulties with crashes and starting the game at the moment, so hopefully Capcom will provide a fix soon.
How to start Monster Hunter Rise: Sunbreak
Since Sunbreak unlocks Master Rank, which is above the regular Hunter Rank in terms of difficulty, you'll actually have to complete most of Monster Hunter Rise in order to unlock the expansion. The specific quest you'll have to finish is "Serpent Goddess of Thunder" a 7-star hub quest where you fight the Thunder Serpent Narwa elder dragon for the first time. This isn't actually the final Rise quest in terms of story, but it's pretty close to it.
Once you've completed the quest, head to the gates of Kamura to find Rondine the Trader. She'll explain how unusual monsters are invading the region, and will send you off to hunt the Daimyo Hermitaur in the Shrine Ruins. Defeat it, and afterwards you'll finally be able to head to Elgado Outpost for the first time.