Most section of this knowledge base involve custom codes, that need to be implemented either in your theme/child theme functions.php file or via a plugin.
Before starting with any of the listed methods, make sure to have a full website back-up just in case. You never know what you may accidentally delete or change.
Method #1 - Theme or Child theme functions.php file
This method is recommendend, as it is safer and more compatible with most custom codes.
Optimally, you should use a child theme. Child theme files does not change during theme updates, leaving the changes in tact.
Open up the functions.php file in your active theme/child theme directory via an (s)FTP file editor. Notepad++, Filezilla and WinSCP are great tools for that. We do not recommend using the theme editor that wordpress offers, for safety reasons. Tutorial here.
The file is usually located at wp-content/themes/your-current-theme/functions.php
Place the recommended code snippet at the bottom of that file.
Save, and it is done
Method #2 - Via Code Snippet plugin
This method is much easier, most snippets should work all right with it. If not, then please use method #1 above.