I generally use this https://wordpress.org/plugins/autodescription/, there are other SEO plugins as well but I won’t mention the most popular to provide another link to it.
The tutorial you’re looking at will work if you remove title support from your theme. Somewhere in your theme you should find a line that looks like add_theme_support( 'title-tag' );, comment it out and WP might stop adding the title tag.
You can also look at the seo plugins and see hoe they do it. I’m not sure and this is more of a general WP question and an ACF question.
