Automating "more" function for Twenty Ten 1.2

I am on WP 3.1.3, using a Child Theme for Twenty Ten 1.2.

Does anybody know of a way to code the Child Theme so that the <!--more--> function is AUTOMATICALLY added after a pre-decided number of words… i.e. instead of doing it manually with the “more” button?

My goal is to have summaries on my front page that include the image I’ve EMBEDDED into each post… exactly as it does when you place the <!--more--> command into a post by using the “more” button.

BTW, I’m new to code and using a Child Theme. Right now it only contains the style.css (correctly coded). Including all steps from there would go a long way towards helping me get it right.

THANX!