Take notes

Whether your best friend moved away or will just be away for a while, writing a paper letter sends the message that you care about them. Even if you live next door to each other, writing a letter can still be a great way to express your appreciation for someone. You can make your letter special with pretty stationery and thoughtful statements. Plus, your best friend can keep your letter as a memento of your friendship and read it anytime they need a reminder how much you care about them.
Take notes.
It can be scary to sit down to a blank page and try to write a long, meaningful letter. Even though you know your best friend well, it can still be difficult to think of what to write.
As you go through your day, take notes of little things you think your best friend would like, events you want to tell your best friend about, or just things you find funny. Your friend will appreciate that you include these little details about your daily life in the letter.
You can do this in a physical notepad, but it can be convenient simply to keep notes in your phone using a note-taking application with a section titled "Best friend Letter."[1]
When you sit down to write your letter, look back over your notes to get ideas of what you want to include as you write.
Think of questions.
When you write, you want your best friend to know that you’re interested in them. Even though you’re best friends and likely have been for a while, there’s always more to learn about each other. Ask them questions, and let them know your answers to those questions. Fun questions you could ask include:
"If you were an animal, what animal do you think you’d be, and why?" You can even tell them what animal you think they are like.
"If you could meet one fictional character in real life, who would you want to meet?"
"If you could choose to magically have any ability or skill, what would you want to be able to do?"
"Do you think aliens are real?"
"What makes you happiest?"
"What’s a food that you absolutely hate?"
"Who do you have a crush on right now?"
"What’s your favorite thing that we do together?"

Make plans for when you see them again.

These plans could be specific or just general fun things you’d like to do together.[3] Having plans written down will make you excited to see your friend and do new things together.
Plan to marathon your favorite movies.
Decide you’re going to have a two-person book club.
Think of a project you’d like to do together.
List out places or attractions you’d like to visit.
Tell them what you like about them.

Sometimes, when you have a best friend, you’re so comfortable with each other that you don’t have to talk about your friendship, it’s just a given. A letter can be a nice way to express the things you always know but never say.

Think about the qualities you admire in your friend.
Also, think of times that they helped you or made you feel better.

Choose your stationery[4] Using pretty stationery can make your letter seem more special. Think about things your friend likes. For example, if your friend loves daisies, try to find stationery with a border of daisies. Get a corresponding envelope as well.

If you choose unlined stationery, you can lay a piece of lined paper underneath to help guide you as you write.
Date your letter. Just write the date on which you write your letter in the top lefthand corner. This will help your friend remember the occasion on which he or she received your letter.
Also, if your letter is delayed for some reason, your best friend will know when you wrote it.
This is especially important if you’re sending the letter far away.
Start with a greeting. This is the standard format for any kind of letter.[5] Begin your letter with "Dear Jane." Use the name of your best friend.
Then, for your greeting, you can simply start with "Hello!"
Or, if you want it to be slightly more personal, try "Hello best friend!" or "Hey BFF!" or use a nickname that only you call your best friend.
Write the body of your letter. Write as much or as little as you’d like. Even if you only write a paragraph or two, your friend will still be happy that you wrote them a letter.
Don’t be afraid to use more than one sheet of paper.
In this part of your letter, include your funny anecdotes and ask how your friend is.
Write them an update on your life. They’re your best friend, and they want to know what’s going on with you. For example, if you have a new crush, let them know.
Give them a list of things they might want to check out, like songs or TV shows.
Write your closing lines. You want to end your letter with something sweet. If you haven’t seen them in a while, let them know you miss them.
Say something like, "I love you! Write back soon!"
End with "Your best friend," or "Love," and then write your signature.
If you forget to write something in your letter, you can always go back and add a P.S. with the message you forgot.
Decorate the letter. To make the letter even more personalized, consider adding little drawings to it. You can even draw on the envelope. Use colored pencils or markers in all your best friend’s favorite colors.
Draw a little picture of you and your friend, or add hearts and flowers.
If you don’t want to draw, you can also use ink stamps or stickers to decorate the letter.
Add a spritz of your perfume. This will sweeten up your letter, and remind your best friend of you. Hold a bottle of perfume several inches from the letter, facing it directly. Spray a few times over the page, but don't soak the paper.
A little bit will go a long way.
Sniff the paper to make sure you can smell the perfume.