Codes, ciphers and computers
Substitution ciphers are easy to break because the same character in plaintext is always written using the same ciphertext.
If the message is long enough you can start to make a good guess to start to break messages based on the frequency of letters. So, in English E is the most frequent letter, whilst X, Q, Z and J are infrequent.
You can use simple tools like spreadsheets to help.
And you can use Python.