A simple way to randomise the letters is to use a password. Remember. the authorised reader of the message needs to jumble the letters in exactly the same way, so any old randomisation will not work.
Suppose we think of a password, say "messup" and use this to jumble up the text. What we do is to write the coded alphabet starting with the password, and omitting any repeated letters. So we get:
"mesupabcdfghijklnoqrtvwxyz"
If this isn't jumbled enough for you, you can write: