How can I start New Game +?

Upon completing the game, interact with a door/gate at your home base. You can Start New Game + at anytime after completing the game

What can you carry over?

You can carry over basic features such as level, weapons, items.

Warning: If you want to access the Fiery Oblivion Depths from the Hellfire Knight DLC, you'll need to have progressed to the last area of the game, even on New Game +. Depths that you have unlocked will not carry over into New Game + and you will have to unlock them again!