I simply want to create a blank HTML file called "left.htm" and another one called "right.htm." I can then edit those two files myself, but I need to know where I am supposed to change the main.tpl and style.css files. Thanks.

If you stored these files in templates/default/ you can just include them like this (in main.tpl):

<!-- HTML ... -->
 
{include file="$template/left.htm"}
 
<!-- HTML ... -->
 
{include file="$template/right.htm"}
 
<!-- HTML ... -->