The easiest way of editing the child theme’s functions.php file to add your custom code is to:
- Open Appearance > Theme Editor;
- Select Rey-child in the top right select list;
- Click on the Theme Functions.
- Add your custom code in the editor.
Please make sure the code is valid and doesn’t have errors. Otherwise you can lockdown your site. Normally the safest way is through FTP or cPanel’s File manager, however if you know exactly that your code is valid, please proceed.
If the basic method described above fails for whatever reason, you can try using some of these plugins: