Correct way of using BEM methodology?

Hello front-end wizards!
I want to take a shot at using the BEM methodology, but I need some help with the following naming.

I made a hero component for my guestbook and I'm not sure how to use BEM here.
Which would be correct, maybe with some elaboration?

hero hero--corners
guestbook guestbook__hero--corners

Or something else?
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