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:
[woocommerce_currency_switcher_drop_down_box]
