Frequency analysis is an amazing tool that we can use to decode monoalphabetic ciphers. In each language, letters of your alphabet are likely to show up a certain amount of times. For example, the letter "E" in the English language is the letter that shows up most often. In a large enough sample, "E" shows up about 12.4% of the time. So if I kept a running tally of all the letters of the ciphertext and found "g" shows up most often (or around 12.4%), I'd guess that "g" decodes to "E".
After calculating the frequency you will see a distribution of the letters of the ciphertext. Compare these percentages with the expected distribution of the English language letters. You will see a dropdown under that particular ciphertext letter. Choose a letter and the page will replace the ciphertext letter with your choice. Continue doing this until you assigned all the letters.
NOTE: If your ciphertext sample is not that large, frequency analysis will not do a great job. That's because the distribution of the ciphertext letters won't necessarily match the expected distribution. There will be a lot more guessing on your part.
Decrypt the message by making substitutions by mapping a letter of the ciphertext to a letter from the plaintext.
After you completely decode your message, check to see if you see a pattern in the ciphertext table.