You can reach it as usual by going around the Sellia Swamp or getting teleported to the cave via a chest in the underground room of the Dragon-Burnt Ruins. Once you enter the Sellia Crystal Tunnel ...

Now, you must travel to the Dragon-Burnt Ruins in Limgrave. It's on the extensive body of water in the middle of this zone. As you get here, go to the exact location marked on the map below to ...