Design your letters as homages to your targets. Make everything you write come back to them, as if they were all you could think about—a delirious effect. If you tell an anecdote, make it somehow relate to them. Your correspondence is a kind of mirror you are holding up to them—they get to see themselves reflected through your desire. If for some reason they do not like you, write to them as if they did. Remember: the tone of your letters is what will get under their skin. If your language is elevated, poetic, creative in its praise, it will infect them despite themselves. Never argue, never defend yourself, never accuse them of being heartless. That would ruin the spell.
A letter can suggest emotion by seeming disordered, rambling from one subject to another. Clearly it is hard for you to think; your love has unhinged you. Disordered thoughts are exciting thoughts. Do not waste time on real information; focus on feelings and sensations, using expressions that are ripe with connotation. Plant ideas by dropping hints, writing suggestively without explaining yourself. Never lecture, never seem intellectual or superior—you will only make yourself pompous, which is deadly. Far better to speak colloquially, though with a poetic edge to lift the language above the commonplace. Do not become sentimental—it is tiring, and too direct. Better to suggest the effect your target has on you than to gush about how you feel. Stay vague and ambiguous, allowing the reader the space to imagine and fantasize. The goal of your writing is not to express yourself but to create emotion in the reader, spreading confusion and desire.
You will know that your letters are having the proper effect when your targets come to mirror your thoughts, repeating words you wrote, whether in their own letters or in person. This is the time to move to the more physical and erotic. Use language that quivers with sexual connotation, or, better still, suggest sexuality by making your letters shorter, more frequent, and even more disordered than before. There is nothing more erotic than the short abrupt note. Your thoughts are unfinished; they can only be completed by the other person.