In the sense that RSA and other more-complicated ciphers don't really apply here, you're correct. But this is the bread and butter of ciphers! Substitution ciphers are always good.
You start simply - tally up the letter frequencies in the passage. If you're lucky, they won't be randomly distributed (some ciphers do such things; of course, we've been told this is a substitution cipher, so it won't unless the message is really, really specially crafted and... weird). While you might hope that you can just plug in the most common letters (which you can remember because they're the letters in SENORITA, but in a different order) for the highest frequencies, in order, that might not always be the case.
But you should definitely try ESAN in for the top 4 frequencies. It's not cut and dry - you'll do a bit of guesswork here, a bit of guesswork there, backtrack a little. Get you hands dirty.
So, to be short: find the letter frequencies in the passage. See what happens if you assume the most common letter is E, the most common letter in a general passage, and so on. Maybe flip a few around. As a general rule, the longer the passage, the closer the actual letter frequencies will be to the expected frequency.