SEO friendly URLs are created by the links on your pages.

It is those links that define URLs.

Once you have links, you then create a rewrite, the pattern of which matches various incoming URL requests.

The rewrite then connects the request to the script and the script serves the content. It is important that the script sends the right HEADERS in response to those requests.

For a page where content exists it should send '200 OK', and where there is no content to send it should send '404 Not Found'.

For malformed requests, either a 404 Not Found response, or a 301 redirect to the correct URL, should be employed.