You need to first create a new BQE ePayments account or use an existing account and connect to it from Global Settings > ePayments. Then you need to define the online payment settings there. After the setup, the ePayments option is enabled for all new invoices, but you can change these options at the client, project and invoice level. You can enable this option for existing invoices from the Invoices screen. Check CORE Help Center > Invoices for details.