WordPress.org

= The really easy way=

1. Go to Plugins > Add New in your WordPress admin panel
2. Search for "barcode posters" in the plugin directory
3. Click "Install" for that plugin in the search results
4. Click the big red "Install Now" button.
5. Go to your Plugins -> Installed screen in the WordPress administration and Activate the Barcode Posters plugin.
6. Go to the Appearance -> Widgets screen and drag the Barcode Posters widget into your sidebar.

Easyish way

1. Download the zip file
2. In your WordPress admin panel, go to Plugins > Add New > Upload (tab)
3. Upload the zip file
4. Go to your Plugins -> Installed screen in the WordPress administration and Activate the Barcode Posters plugin.
5. Go to the Appearance -> Widgets screen and drag the Barcode Posters widget into your sidebar.

The hard (FTP) way

1. Unzip the zip file somewhere. This gives you a folder called barcode_posters_widget.
2. Upload that folder using FTP to the wp-content/plugins folder on your web server.
3. Go to your Plugins -> Installed screen in the WordPress administration and Activate the Barcode Posters plugin.
4. Go to the Appearance -> Widgets screen and drag the Barcode Posters widget into your sidebar.

If you're using a caching plugin such as WP-Cache you may have to delete your cache before the new sidebar widget appears.