Adding More… to Classic theme

I’m trying to add the tag “more” into my text so I can abbreviate the post and have it continue on a full page. Reading in the forums here I’m not sure the WP Classic theme can do that without adding in code. Is that true? If so, what file and where do I put the code? And how do I customize it? I’ve looked at the WP support for changing the tagline, but it was written for an earlier version and doesn’t correspond. I’m using WP 2.2.3.

Thanks for any help.
Ed