Perhaps 'paper' is better than 'digital' in this case.
The history of love letters extends all the way back to the beginnings of civilized correspondence. Love letters from ancient Egypt, Imperial China, and William Shakespeare's sonnets have become good examples of how to write on emotional themes.

"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;
- My mistress' eyes are nothing like the sun" —William Shakespeare

Before starting to write, make sure that you create a perfect atmosphere that will get you in the mood and give you some ideas on what to write about. Go to some private room, put on some light romantic music, and place a picture of your loved one in front of you to help inspire you.

A love letter should always be handwritten rather than typed. Choose attractive paper or stationery and deliver it in person or by mail. Doing it this way, you are showing the object of your love that you care enough to take the time to write it out, which adds a personal touch. If you must type the letter, choose the most suitable font that will look romantic, such as Rage Italic or Randy, sized to resemble handwriting.
At the top right corner put the date. If the letter took days to complete, then just put the date of completion. You can address the person as Beloved, Dearest, Beautiful, Most Cherished, or, if you are already in a romantic relationship, you can say To My Dearest. For a relationship that is just beginning, choose something detached like To my lovely.

Livy darling,
Six years have gone by since I made my first great success in life and won you.
—Mark Twain to his wife Livy on her thirtieth birthday

At the very beginning of the letter make sure that you point out that it is a love letter and not a simple note or a card.

My angel, my all, my very self—only a few words today and at that with your pencil—not till tomorrow will my lodgings be definitely determined upon—what a useless waste of time. Why this deep sorrow where necessity speaks—can our love endure except through sacrifices—except through not demanding everything—can you change it that you are not wholly mine, I not wholly thine?
Immortal Beloved, Ludwig van Beethoven

Further on, describe your feelings. Explain the physical and emotional satisfactions that the object of your love is causing within you. Try recalling the moment when you were most touched by that person’s gestures or words and describe in detail how you felt. Show that all your attention was anchored on him or her. That is something that will please most lovers.

I awake full of you. Your image and the intoxication of the last night give my senses no rest. Sweet, incomparable Josephine, what a strange effect you have on my heart. Are you angry? Do I see you sad? Are you worried? My soul breaks with grief, and there is no rest for your lover.
—Letter to Josephine, Napoleon Bonaparte

In the second part of the letter, explain the virtues of the person that you are addressing, such as kindness, sense of humour, intelligence or beauty. Avoid focusing only on appearance, and let this person know that you appreciate them on a number of levels. Use real details and in your own words, instead of the same old cliché expressions such as "Your eyes are like stars" or "red lips like roses". Express your emotional declarations one at a time, and make sure not to repeat yourself.

But nothing sounds as precious to me as the ringing sound of your darling name. I won’t drink poison, or jump to demise, or pull the trigger to take my own life. Except for your eyes, no blade can control me, no sharpened knife.
—Love Letter to Lilya Brik, Vladimir Mayakovsky

End the letter by explaining what kind of relationship you hope to have with him or her, bearing in mind that the reader can sense the deep, true feelings of the writer. Write about the future that you are planning to secure for both of you, but stay within the realm of reality. All in all, be as realistic as love allows you to be.

When you are old, I want you to recall those few hours, I want your dry bones to quiver with joy when you think of them.
—to his wife Louise Colet, Gustave Flaubert

And do we have to mention that, before delivering your letter, you should check it for sentence structure, grammar, and spelling mistakes?