Upload holler.php to your plugins folder, usually wp-content/plugins/
Activate the plugin on the plugin screen.
Use the Wordpress plugins editor and set the addresses you wish to send to.
Add the following line to your templates where you want the contact form to appear:
If you want the form to appear in only one post or page, add where you wish it to appear. Be careful to only include one instance of holler in each page, or you'll get unpredictable results.
To localise Holler, download the corresponding .mo file and put it in 'wp-content/plugins/holler'