If your host supports the use of htaccess but not the use of SSI, there is probably very good reason for it. Please check with your host before doing this procedure.
The first line states that pages with an extension of shtml are valid files. The second line adds a "handler" which is the actual SSI stuff. Any file with the shtml extension must be parsed by the server. The third line makes it all mesh together on the server.
That's it. Now any pages you create on your site with the shtml extension will be able to use SSI coding.
What about using SSI on .html extensions?
Don't feel like renaming a bunch of files over to shtml extensions? Then try this instead:
A bit of caution on that last addition though. This will force ALL pages on your site with an html extension to be sent into the server for parsing even if it contains SSI commands or not. The extra time to do the parsing action is transparent to the human eye, but it may bog the server down doing all these extra commands when it isn't required.