Redirect with mod_rewrite for subdomains

[Alfons]

I just startetd to change my old .htm pages to xhtml, which will also change the extension to .html. As external links can no longer find the website, I wanted to create a permanent redirect for each page from www.example/site.htm to www.example/site.html in a .htaccess file.
Obviously this is not working for subdomains and my hosting company told me to use mod_rewrite code and upload it as a .htaccess file. I have no idea how to do this and the information about mod_rewrite in the available websites is not understandable for me. Anyone who can help me in this matter? Thanks. Alfons