Enabling SSI's in Apache requires only a few things:
  1. Compiling in mod_include into the server or using LoadModule to include it as a DSO
  2. having it enabled with the Options directive (i.e. Options Includes)
  3. a method of specifying how the server knows which files to parse

Server Side Includes: A Mini-Tutorial

Note: there are additional features supported in Apache 2.0, the documentation for the updated version of mod_include is here. Among the cool new things are user definable SSI delimiters, user definable error messages and other good stuff. See also the Apache site's SSI Howto.