I would do this in one of two ways.
The first case is where the singing is "mere dialogue" and not particularly important. It's simply something that a character is singing, rather than being an important part of the novel. In this case, I would put the words in italics, and punctuate them as if they were prose:
She suddenly began singing, "Cruella de Vil, Cruella de Vil! If she doesn't scare you, no evil thing will! To see her is to take a sudden chill! Cruella, Cruella! She's like a spider waiting for the kill! Look out for Cruella de Vil!"
The second case is where the words are an important part of the novel, and you want to display them in a distinct and weighty fashion. In this case, I would write the lyrics indented, with no quotation marks, in italics, with line breaks, using end punctuation only sparingly:
She suddenly began singing:
Cruella de Vil, Cruella de Vil!
If she doesn't share you, no evil thing will
To see her is to take a sudden chill
She's like a spider waiting for the kill
Look out for Cruella de Vil.
I would never use slashes for lyrics in a fictional story. For one, I don't think I've ever seen slashes used that way in a published work of fiction. For two, when writing fiction, it's important to keep aesthetics in mind, and in my personal opinion, slashes just don't look very good. When writing non-fiction, it's usually important to let the reader know where the line breaks are, but in fiction, that's probably not important.
Of course, you should think about the legal aspects as well. Personally, I think that quoting a small section of a song in a novel is almost certainly fair use, but don't take my word for it.