Once your league status changes to "Shipped," the Schedule new demo event button will be shown on your League Dashboard. Click that to schedule your event. Enter the date and time for the event, and click Save. There's no need to enter a name, each event will be automatically named.
To navigate to your League Dashboard, click on Pokémon League in the top navigation menu. Scroll down to the bottom of the landing page and click View your Demo Events status. This will take you to your Demo Events Dashboard. Click on the League ID or Store Name to go to that league's dashboard.