If you want to add the code on a specific post or page, search for the required tag or anything in the post and wrap the code in the function section. Now the only thing to remember here is that the "is_single" keyword is used for a post and "is_page" is used for page. Using this way, you can hook your code to both wp_header and wp_footer file. You can also add conditional tags on the specific post or pages as well.
You don't have to access the backend files or any other thing, just install the plugin and paste the code. Download the plugin and activate it from the dashboard. Now go to the "Screen Options" and check the "Custom Fields" option. Now enter a new custom field in any of the posts/pages and start adding the values.