Use a WordPress Plugin
WordPress Dashboard > Menu Item: Appearance > Sub-Menu Item: Editor.
When you get to your editor screen, look for your Theme’s Footer (footer.php). Click that.
Now your Theme’s Footer (footer.php) should be showing up. Go to the code area below
<?php wp_footer(); ?>. Below that, but above the closing
</body> tag is where your code should go.
Therefore, it should looking something like this: