Within your Time Tracker account expand the More tab and select Integrations. Once you've landed on the integrations page select QuickBooks Online.
From here you can choose if you want to include the Payroll integration. After you've chosen, click on the Connect to QuickBooks button.
An Intuit window will appear. You'll then be prompted to sign in with your Intuit account. The login credentials you use here should be the same as the one you use to sign into QuickBooks Online.
Authorize your connection with Time Tracker by clicking the "Authorize" button. Time Tracker will then show additional screens asking to link up entities between QuickBooks and Time Tracker. Once that is completed you should be all set!