If you already have an erplain account and want to connect to your QuickBooks account, simply go the erplain Menu: 'Apps':
Click on the 'Connect to QuickBooks' button:
Click on 'Authorize':
Your erplain account is now connected to your QuickBooks account.
Please note that you need a QuickBooks Online Plus account to sync products, invoices and purchase orders from erplain to QuickBooks.