Defeat the three Lords of Cinder
Upon defeating all three Lords of Cinder: Abyss Watchers, Yhorm the Giant and Aldrich, Devourer of Gods you will get immediately transported back to the Boreal Valley.
Place the plate under the statue
Once you receive the plate from the dying lady, place it under the statue and you’ll get in a cutscene where the Dancer will descend and a fog gate will cover the gate.
The Dancer will descend from above
Dancer of the Boreal Valley will descend from the entrance gate and you will have to fight them.
Make sure you’re light
Make sure you’re light on equipment because you will need to evade almost immediately.
Dodge one attack
Dodge one attack and get behind the dancer. If you hit it a number of times rapidly, it will stumble and be open from a critical attack. (See Dark Souls 2: Scholar of the First Sin Boss Fights Guide)
Avoid her in the second phase
If you manage to avoid Dancer of the Boreal Valley during the second phase of her attack, you’ll be able to defeat her almost immediately.
Dancer of the Boreal Valley will drop 60,000 souls and Soul of a Dancer. (See How to Defeat the Curse-rotted Greatwood – Dark Souls 3)
How to Beat Dancer of the Boreal Valley in Dark Souls III
Dark Souls 3 Boss: How to Beat Dancer of the Boreal Valley