Explain this quote from "A Wrinkle in Time": "You are given the form, but you have to write the sonnet yourself."...What you say is completely up to you"?
It may be helpful to see all of what Mrs. Whatsit says about the sonnet:
A sonnet is a very strict form of poetry is it not? There are fourteen lines, I believe, all in iambic pentameter. That's a very strict rhythm or meter, yes? And each line has to end with a rigid rhyme pattern. And if the poet does not do it exactly this way, it is not a sonnet, is it?
Then Charles says, "You mean your comparing our lives to a sonnet? A strict form, but freedom within it?" That's when she says the sentence you're asking about.
What she is really talking about is free will or freedom of choice. Our lives may be subject to a higher power (in L'Engle's belief, God), but our lives are not predetermined. Like the poet writing a sonnet, we are subject to rules and forms, but it is up to each one of us to choose how we will live. A major theme of A Wrinkle in Time is the battle between good and evil, so one of the choices we make with our lives is whether to be good or evil.