If you created an HTML5 banner already, you can select your banner using the Show Existing Creatives button. Otherwise, you can also upload your own banners that meet the specifications by Clicking to Browse or Dragging and Dropping into the box.

When that’s all finished, just click the pink Publish button at the bottom of the screen. And you’re all done! The status and progress of your ad can be monitored in the Ad reporting page, which you can navigate to from the home screen.

Keep the Create follow-up remarketing audience checked to enable future remarketing campaigns that can target all of your impressions generated from this first ad campaign.

See here for further info.