You won't find the Corrupted Monk until fairly late in Sekiro: Shadows Die Twice, once you've ventured into the misty Ashina Depths and crossed through Mibu Village. After dealing with a forest full of apparitions that will attack you along the way, you'll finally come to the ghostly boss. Its long reach and high Vitality make it a tough fight, one whose rhythm you'll need to learn in order to survive.
The biggest threat about the Corrupted Monk is how long its weapon is, which makes the arena where you face it a bit tight. The Monk likes to do a lot of lateral attacks you'll need to parry, while also throwing in a lot of spinning blows that can push you into corners. Do your best to stay in the center of the area, and capitalize on your chances to attack the Corrupted Monk. Your best shot is to counterattack right after the Monk does its leaping vertical slice, which you can either dodge or parry.
Knocking down the Vitality of the Corrupted Monk is the key to winning, since it'll recover its Posture quickly otherwise. That means you'll need to do damage when you can by getting in close. Check out the video above to see how best to handle the boss, as well as what to avoid. Note that you can do a whole lot of extra Posture damage to the Corrupted Monk if you use Divine Confetti, which wrecks apparition enemies like this one.
We've got plenty more Sekiro guides for your perusal, including rundowns on how to beat Lady Butterfly and the Guardian Ape, how to deal with Dragonrot, and where how to activate Hard Mode using the Bell Demon.