Head to Settings > Integrations
Navigate to Big Cartel and click Connect.
Follow the flow and log in to Big Cartel as prompted.
Once completed you'll be brought back to Openstage and see the below.
If your connection has been made you'll also see the below image in the top right corner of this page.
You can find fans who have come in through this channel by searching for the tag 'bigcartel-welcome'. You will also find this data in the Transactions section of your account.