Use the plugin back-end to add and manage custom codes
IMPORTANT: Make sure the snippet is set to run everywhere
Theme or Child theme functions.php file (Advanced)
This method is safer as you have direct access to the files and any issues can be undone quickly. However switching the theme will also turn off all custom codes, as they have to be carried over to the new theme.
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.