First there's doing it enough to avoid copyright violations. And possibly accusations of imitation.
But as the muse works on, often, while the writing goes on.
I've got a character being raised by his guardian after his parents were killed in a magical attack that mysteriously didn't kill him, which makes him. (Also known as what Harry Potter's life might have like had anyone had the brains to use anything of their wonderful spells on Sirius Black -- with a few additional changes. 0:) And plugging along and he's telling his classmates about it -- and the muse starts burbling about how the attack could be different even more than its being a mindless creature that certainly was killed in the process. . .
Now. After I'm already writing.
It might even be a good idea to do it.