There are many examples out there - both in this forum and others.
The goal is to enable SMS reception, and accept +CMT messages.
Google is your friend... that’s how I learned, along with hours of experimentation & practice.
Suggestion: When you find examples, DON’T use Strings —- use C-strings to avoid unstable operation with a small memory processor like Arduino.
c-strings are a learning curve, but offer performance and flexibility benefits when you’re handling character strings.