Retrieving AN API key
First of all you need to register a new client here.
In the first tab enter all required fields. In the "Valid redirect URIs" field paste in the URL that is mentioned before.
In the next tab just uncheck all checkboxes.
Click the "Register" button. In the next page you can copy your Instagram Client ID.
In the last step you need to make your APP live, otherwise it can only be used by you. In the "Edit Client" screen you find a green button "Go Live", click that one and your APP is live.
Entering the API key
The correct redirect URI can be found in the General Settings of Fancy Product Designer. Just copy it and paste it in the Redirect URI field. Last you only need to enter the Client ID in the Instagram Client ID option field. If your site is running under HTTPS, be sure that also the redirect URI is stored as HTTPS. Therefore you can easily edit the option field in the settings. Use also this HTTPS URI as Redirect URI in the Instagram configuration.
The redirect URI and Client ID need to be set in the plugin options to enable the Instagram pictures. The redirect URI is the URI to the html/instagram-auth.html, e.g.