+1. Edit your `.setup` file and add `404` to the `add_plugins` line.
+2. Add a 404 error document handler in your Apache configuration:
+
+ `ErrorDocument 404 /url/path/to/ikiwiki.cgi`
+
+ Where `/url/path/to` is the path portion of the URL to the `ikiwiki.cgi` binary.
+
+This plugin might also be useful on non-Apache web servers, if they provide the
+`REDIRECT_STATUS` and `REDIRECT_URL` environment variables to their 404 handlers.
+`REDIRECT_STATUS` should be `404` and `REDIRECT_URL` should be the path
+part of the URL (for instance it would be `/plugins/404/` if this page was missing).
+
+If you would like help with adapting this plugin for a different web server,
+you will need to provide the output of
+[[this 404 handler|forum/nginx:_404_plugin_not_working#comment-6b1607f7961d2873517d4780f56ac3ad]].