I would also recommend using an existing sonnet as a template. With my students, I have them write a sonnet in the style of Sonnet 130. Because it starts off so seemingly insulting, my students seem better able to sink their ink into the task.
I have them follow the form and style, and try for iambic pentameter. If that's too hard, just count out ten beats a line. Then use a similar rhyme scheme (this one is ababcdcdefefgg). The main point is in the rhyming couplet at the end, and it is a reversal of the original point--she may not meet any of the conventions of beauty, but she's rare and she's real.
SONNET #130 By William Shakespeare My mistress' eyes are nothing like the sun;
Coral is far more red than her lips' red;
If snow be white, why then her breasts are dun;
If hairs be wires, black wires grow on her head.
I have seen roses demasked, red and white,
But no such roses see I in her cheeks;
And in some perfumes is there more delight
Than in the breath that from my mistress reeks.
I love to hear her speak, yet well I know
That music hath a far more pleasing sound;
I grant I never saw a goddess go;
My mistress, when she walks, treads on the ground.
And yet, by heaven, I think my love as rare
As any she belied with false compare.
You can keep them theme the same and change the comparisons but keep much of the original structure. Be creative and have fun.