To create your payment methods in erplain, please find the instructions on this page: Payment methods.
Once you have created your payment methods in erplain, you can map your payment methods with an QuickBooks ledger account in the menu Apps > Configure QuickBooks > Payment methods:
Once an invoice as been synced to QuickBooks, the status of the invoice will be "Sent to QuickBooks" regardless of the payment status.
If you receive a payment in erplain, it will be synced to QuickBooks, and the payment will be assigned to the mapped ledger account.
If you create the payment in QuickBooks instead of erplain, the payment will not sync back to erplain but you can still check the QuickBooks status when you open the invoice in erplain:
Typically, customers receive payments in erplain so that it shows in both systems. In some cases, if you prefer to receive the payment in QuickBooks, this is not a problem but the payment will not show in erplain, only the QuickBooks status when you open the invoice.