Write down a list of the 26 letters of the alphabet, along with numbers zero to nine. Leave some space next to them to insert their symbols.
Place a letter, number, or symbol next to each letter and number. It is your choice whether to just reshuffle the alphabet's letters, use numbers for each letter (for example 1,549 for "B" and 4 for "Z") or use symbols, such as the dollar sign ($) or zodiac signs. Letters from the Cyrillic and Greek alphabet are also useful for this purpose.
Use one or two additional symbols for each letter, to confuse those aiming to decipher your letters. For example, "A" can be "j," but also "Θ" and "989." Receivers of the letter must check all three possibilities before deciphering the letter.
Place specific symbols for certain couples of letters. For example, "E" can be "k" and "X" can be "q" but when placed together (in "EXample" for instance), you won't write "kq" but "?" (from the Cyrillic alphabet). This way, you confuse outsiders even more.
Replace punctuation marks with symbols resembling the rest of the code. For instance, put "Σ" for the period, so that illegitimate readers cannot instantly realize that this is where the sentence ends and that it is not another letter.
Copy the code and distribute it to your friends or relatives. Make it absolutely clear that they must keep the "dictionary" a secret, otherwise the code becomes useless.