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...

It may be
helpful to see all of what Mrs.
Whatsit says about the sonnet:

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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  is the battle between good and
evil, so
one of the choices we make with our lives is whether to be good or
evil.