So pay attention to your words. Remember, it’s “scent,” not “odor.” Your beloved doesn’t “smell” good; her “fragrance” is enchanting.
If you find yourself stuck, begin with a quotation. Shakespeare is a safe bet, especially his Twelfth Night, in which one woman, disguised as a man, woos another woman on behalf of the actual man the first woman secretly loves. Yes, it's complicated, but you can learn from the Bard’s play how a woman might woo if she were a man—an invaluable lesson in imagining what the object of your affections wants to hear.
And, even if you have a knack for them, no pornographic drawings.
Ulysses S. Grant peppered his love letters to Julia Dent with blank spaces, which, he was forced to explain to the baffled lady, were an attempt to suggest feelings that words could never express. It worked for Grant, who married Miss Dent after four years of courtship. It might work for you.
Use metaphor, not euphemism. If you don’t know what a metaphor is, rent Il Postino. In the film, the exiled poet Pablo Neruda explains the concept to his postman, and the metaphors invented by Mario, the tongue-tied mailman, win the heart of his gorgeous Beatrice.
Neruda's own poetry is also an invaluable trove. His book Twenty Love Poems and a Song of Despair is full of examples (and its final song may prove of comfort if your letter fails).
A few rules apply. No cute goofiness. No financial metaphors, particularly employing the conceit of what an excellent investment your lover is. Food is a good choice, but be careful. Fashionable vegetables may be all the rage at that little bistro where you take your beloved after a film like Il Postino, but a fiddlehead fern may be more attractive on the plate than on the page.
If your lover is female, you can compare her to a flower. Jorge Luis Borges, always intrigued by labyrinths, reminds us that one of the immortal metaphors of poetry pairs a woman with a flower. You should realize, though, that roses (and oysters, for that matter) are associated with love in part because of their physical resemblance to a particular part of a the female anatomy. When Bobby Burns sings, “O, my luve’s like a red, red rose,” he’s speaking literally as well as figuratively. So consider carefully all the implications of the metaphors you strew.
Fernando Pessoa, the great Portuguese poet, insists that immortality depends upon the grammarians. He knows what he’s talking about.
Consider the case of Confederate officer William F. Testerman, for example, who penned these concluding sentences to his beloved: “Direct your letters as before and dont forget your best friend so I will end my few lines but my love to you has no End remember me as ever your love and friend. Excuse bad riting.” Perhaps Miss Jane Davis, to whom the soldier's letter was addressed, forgave his prose. He did, after all, write from the battlefield. But you, in composing your love letter, seek to make eloquent those reasons of the heart most resistant to glib formulation. “Bad riting” won't ease your task.