To start a case with Unpaid in Billit, go to 'Settings' > 'Integrations' and 'Unpaid'. If you have never created a case with Unpaid before, you will see the 'Available' button in the tile. Click it to create your first case. Now click 'Start new case'. You will see a list of all overdue B2B invoices. Select the correct invoice with '+ start new case'. Then follow the further steps at Unpaid.
After you have started a case with Unpaid for the first time, the button on the integrations page will automatically change to 'Installed'.