Set up your webserver to enable mod rewrite
This step really depends on what webserver you are using and how it has been configured previously. There are lots of tutorials on the web about this and we suggest you google for them.
A couple of things that might catch you out. Firstly you need to ensure mod_rewrite is installed and turned on in your webserver conf.
You also need to ensure that there is a directive either in your main httpd.conf, your virtualhost file or your .htaccess file (depending on where you are setting up your Directory directives) that sets AllowOverride to All.
Also, don't forget to restart your webserver after any configuration changes!