Child Theme, How to

Hello, and Help!

I have already used the theme and made some changes in some of its files (content.php, header.php, footer.php, functions.php, etc.) mostly for translation and some functional needs. Now I want to create a child theme but I don’t know exactly what to do. I tried to copy all files changed into the ‘child’ directory but the server returned the error regarding problems with functions.php file.
I read some articles on creating a child theme but all of them explain how to work from the very beginning with untouched parent theme.
What should I do to make a child theme to work and to save all the functionality and look of parent one?

Any suggestions welcomed. Thank you.