Navigate to your site's Admin interface (this is specific to your shop, i.e., we don't know it - it should look similar to: www.yoursite.com/wp-admin), and login to the Admin interface.
Click on the 'Plugin' menu item on the left-hand side of the page.
Click 'Add new' in the upper left hand corner.
In the same location as Step 4 you should see 'Upload Plugin' - click the button.
Follow the steps, and upload the file you downloaded inStep 1.
On a successful upload WordPress will ask you if you want to 'Activate Plugin', or 'Return to Plugin page'. We wish to Activate the plugin.
The settings menu for Expressly is located as an additional tab to the WooCommerce settings page. This page can be found by pressing 'Settings' under the WooCommerce title on the Plugin page (you should be redirected here after step 7) [For reference, the URL would be www.yoursite.com/wp-admin/admin.php?page=wc-settings].
Place the API Key into the field provided, and press save. You should receive a message telling you that everything has been completed successfully, and you should be ready to go!