Modifying css and php and adding in php
I am fine with css and php but cannot work out how to create the child theme and achieve what I want to achieve.
1. I want to edit css from the original code
2. I want to edit php from the original code
3. I want to add php into the original code
4. I want to call new php files from the original code
5. MOST IMPORTANTLY - I want to be able to upgrade with minimal disruption, searching for changes, problems ...
Thanks in advance if anyone can help or point me in the right direction. Ps The child theme info on the site does not seem descriptive enough to do the above without messing it up somehow