In the age of text messaging and rushed emails, a good old-fashioned letter, especially a handwritten one, is a rare and special treat. To write a love letter, follow these steps:

1. Choose nice paper or stationery. Don’t email a love letter; give the person something they can touch, feel, and, if you’re lucky, tuck into their pillow at night. It's best to write on plain paper that has either a simple (for example, white), calming (for example, cream), or sensual (for example, flesh-toned) color.
- While a simple piece of printer or notebook paper will do, selecting high-quality paper will add a nice touch and show how much care went into writing this letter.
- You can make regular paper look old or even make your own paper.
- Use either a black or brown ink to keep the writing grounded and classy-looking; avoid “teacher colors” like blue, green, and red that will make it look like you’re marking up a homework assignment.

2. Set the mood. Go to a private room, like a bathroom or bedroom, put on some romantic music, and dim the lights (or even light candles). Make sure you won’t be interrupted. If you have a song or album that reminds you of the person, put it on; it will help you conjure up memories and feelings.

3. Use an intimate salutation. Address the person as “beloved,” “dearest,” “beautiful,” “most cherished,” or, if appropriate, a pet name. If you are already in a romantic relationship, you can say “my” (for example, “To My Dearest ____”), but don’t do this if you’re using the letter to confess your feeling; it will come across as presumptuous and territorial. Instead, use something more detached like “To The Lovely ____,” for example.

4. Think of a time when you felt particularly moved by seeing your loved one, no matter how small the event. Recall the moment in vivid detail; this proves that you were fully present and that all your attention was fixed on this person––something most people love to hear. Remember the emotional and physical feelings you experienced in that moment. Then, write a description of that moment including details about your feelings at the time.

5. Tell the person what you appreciate most about them. Is it their compassion? Thankfulness? Beauty? Sense of humor? Humility? Resourcefulness? How do they make you feel every day? Do you consider this person your best friend? Can (s)he be counted on through thick and thin? Write about a combination of attributes––looks, personality, humor, charm––to let this person know that you appreciate them on a number of levels.
- Be aware that exclusively focusing on appearance will make your feelings appear superficial. However, exclusively focusing on personality might seem too platonic (or make the individual feel unattractive). If appropriate, be sensual (but not openly erotic).
- Be totally, unabashedly honest. Your writing style, spelling, punctuation, grammar, and any other distractions should take a back seat to your confession of exactly how you feel and why.

6. Write about the future. If you’re confessing your feelings for the first time, admit how giddy you are every time you know you’re about to see them again. If you’re apart, describe all the things you want to do together when you’re reunited. If you’re committed, discuss some goals, dreams, and fantasies you have about your future life together.

7. Sign the letter. Suitable sign offs include “Yours,” “Yours Forever,” “XOXO,” “Kisses,” “Love,” and “Love Always.” If applicable, include a pet name, inside joke, or even a reply to a long-unanswered question to make it more personal.

8. Add a personal touch. Put lipstick on and kiss the paper, spray the perfume or cologne you always wear on the paper, or trace your hand on the back so that the other person can hold their hand up to yours.

9. Put the letter in an envelope. If you want, you can make the envelope or even fold the note itself into an envelope. Alternatively, roll the letter up like a scroll and tie it closed with a piece of nice ribbon or string.

10. Write other love letters. Don’t make this a one-time event; make a habit of writing love letters to the person you care about for birthdays, anniversaries, time spent apart, time spent together, or no particular reason at all. The more you do, the easier to write and more meaningful they’ll become.
Source: wikihow.com