That wasn't my code, that was Jackie's code, which I used to answer her coded message. Actulally I tried to make it easier to solve by using "giveaway" words in my message, like I've, I and a, all and see, and such like. And Jackie's name in the reply.

Making up a substitution code is easy: just write down the alphabet, as Jackie did, and write any rearrangement of the letters underneath it, random or selected (the code alphabet might be, for example,

plain: A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
code: J A C K I E B D F G H Z Y X W V U T S R Q P O N M L

The problem in making your coded message, I've found, is keeping straight which set is the code letter and which is the plaintext letter when you finally write your message!