How to do error logging in CodeIgniter (PHP)

I want error logging in PHP CodeIgniter. How do I enable error logging?
I have some questions:

1. What are all the steps to log an error?
   
   
2. How is an error log file created?
   
   
3. How to push the error message into log file (whenever an error occurs)?
   
   
4. How do you e-mail that error to an email address?