Here are a few tips to help you write a timeless romantic love letter.

Avoid the use of slang when possible. This isn’t the time to tell “bae” you love them. Make the words you use count.
Say what you feel but keep it simple. Even if you struggle to express your feelings aloud, take the time to write them out. Speak from your heart, and be genuine. This isn’t going to win a literary prize. It just needs to be authentic.
Avoid clichés. Skip roses are red/violets are blue. Less is more in this case.
Consider your intentions. What do you really need to communicate? Do you want to tell your partner that you’re sorry for something you’ve done? Is your intention to express gratitude? Do you just want to share how much they mean to you? Is this a letter to someone who’s gone or who is still present in your life? Consider the letter’s intent and then speak from the heart.
There’s nothing wrong with a rough draft. If you’re a letter writing novice, take the time to practice what you want to say. Then just say it.
Hand-write this letter (if your handwriting is legible). A hand-written love letter is a beautiful gesture, but you also want your beloved to be able to read what you wrote. If your handwriting is truly awful, type it out, print it, and sign it. Hand-delivering could work if handwriting won’t.
Don’t worry about perfection. The best part about reading famous love letters is being able to feel how much someone was loved. We don’t read them for grammatical errors or mistakes. Someone who cares about you probably won’t pull out a big red pen to start making corrections.
Take some advice from Vincent van Gogh: “Do you know what frees one from this captivity? It is every deep serious affection. Being friends, being brothers, love, these open the prison by supreme power, by some magic force. Where sympathy is renewed, life is restored.” Also: “Love a friend, love a wife, something, whatever you like, but one must love with a lofty and serious intimate sympathy, with strength, with intelligence, and one must always try to know deeper, better, and more.” Simply love someone.
Avoid promises you can’t keep. Be careful with the words you use. Empty promises will take away from the message and can be hurtful later in the relationship.
Borrow someone else’s. If you can't find the words, borrow someone else’s. Quote a poem or a line in a book that resonated so strongly with you that you needed to share it with the one you love. Don’t try to pass them off as your own words, but it still can be impactful to say that this poem made you think of them.
Maybe you’d rather read love letters than write them, but romance isn’t a lost art. We’ve just gotten out of practice. The ease of communication can make it difficult to connect in the same way.

This isn’t to say that texts and instant messages can’t be romantic. They can. Send them a song that made you think of them. Find a poem that says it all. Or just write it out — all your love in words, sent over to them like a gift. It just takes thought, effort, and a little courage to say what we need to say to the ones we love the most.