Go to Magento Frontend - login as customer and place some order
Go to Magento admin panel - Sales - Orders - Click on the Salesforce Opportunity ID link
...
Select the Detail tab and change the following data
Opportunity Owner = Integration user
status = Close Won
...
Click Sync with Magento button
...
go to Salesforce Magento queue and check new Opportunity record in Success status
...
go to Magento Admin panel - Sales - Orders - Edit previously created Order
Go to Information tab - Order & Account Information
...
Sales Representative = integration user
status = complete
...