1. Install

First install the Currency switcher for WooCommerce plugin. It's free.

2. Setup

Set up the plugin within WooCommerce > Settings > Currency Switcher. Within the Currencies tab you can choose which currencies to use.

3. Display

To display it on the site, I find it's easier to use a shortcode. So, go to Appearance > Widgets > Top Bar Right and add the following within the list: