Good morning or Good Night everyone!

I have doubts about the BMO vs. Lady cards.

1 - Dr. Death: Destroy a creature you control.
But how do I destroy it? I attack? It Does it leave the landscape, just like replacing it? I do not know how to destroy a creature of mine.

2- Mace Stump / Green Mermaid:
Destroy ****** >>>.
That is, when my opponent destroys my creature, or is it when I destroy it? (Returning to question 1 of how to destroy my own creature.)

3 - Immortal Maize Wallker:
Is the triple damage effect only used when the opposing monster is in a cornfield?

Thanks for helping!