Hours worked by the user on each project are recorded and saved in Salesforce. To generate invoices as per the hours worked by users on a Project, please add the "Hourly Billing Rate" and "Billable Amount" details in the TT Users screen.
Link Xero Contact with the Project on which the User is working on, once the user Checks-In and Checks-out of the projects, the total hours worked on that project are saved in Salesforce.
Then the scheduler created in Salesforce for generating invoices automatically picks the total hours worked by the user on a specific Xero contact and calculates the hours worked with the billable amount and generates invoices automatically in Xero.
You see the invoices under the Invoices tab which is in the Billing tab in Xero. Accounts people check the invoices, approve them and send them to the clients.