A very old encryption method is the Caesar Cipher. For testing purposes, usually the ASCII code is used. But you can add a little more security to your message.

Rules: Shift all letters by 5

Apply the rules.

One solution: Give the letters a random number, or use some fake numbers in between to confuse the spy.

An attacker (f/m/d) could find some letters due to their "letter frequency".

The letter frequency varies in different languages, books, special texts or in dictionaries.

Some recent child names increase the rare letters.

Now you could use a letter for the "space" between the words, or shift the second word by increasing with one.

Rules: space = R, "shift-start" with 5 and increase by 1 each word.

Now with the new rules.

In case your name is Bob, your mate is called Alice.

Those are the most used names in illustrations. A and B...

The main problems with a symmetric algorithm is that you decode and encode with the same key and you need to send the key together with the message.

Or a second way: The security level of your "key" depends on the safe transmission of your key to a friend. An evil dude should not take a look, in case you send the key with a "secure" postcard or letter.

A real help gives us an asymmetric cryptography algorithm. But that's an other story.