Well, they work with specific sprites depending on which boss you're adding

Considering that you're new (correct me if I'm wrong), I suggest that you just copy paste the area of a boss and edit it...

But, in case you want to know, use these sprites:

Boss: Sprite 58 or 295 or 65 or 21 or 229 or 30 or 47 or 63 or 29 (because Peach is apparently the final boss)
Boss controller Sprite: 39: Castle boss controller or 35: Final boss controller
Key/switch: Sprite 129: Boss key location or Sprite 41: Bowser bridge switch (skull switch)

(Use only sprite 220 for Bowser Jr)

Music:
52: Castle ambient 3