Fixture was built to help you build and manage multiple sites from a single account.  This is all done through a single UI.  To manage your sites, navigate to Settings using the cog icon in the top right menu (see below) and then select Sites.  If you have multiple sites, you can quickly switch between sites just by clicking on the name of the site.  To add a site, click "Add Site" in the top right of the screen.

When you add a site, you will type in a friendly name for the site, select the folder within your bucket to create the site, and the theme for the site.  You must also acknowledge the added cost for a site.  This will increment your Stripe subscription.

You can quickly switch sites using the arrow next to the view site button.  This enables quick management of multiple sites from a single pane of glass.