How to add "read more" tag in post?

Greetings:

I’m using a Spacious child theme and I’d like to split up a long post by adding the “read more” tag within the post. I inserted the tag from the WP default editor but that didn’t work.

From reading the WP codex on this topic it looks like a somewhat complicated process that I didn’t fully understand. Could someone simplify it for me?

Thanks.