You can start the timer on a Project task that needs to be time tracked by clicking on the timer icon on the right top of the Project Task

Follow these steps to start logging time
1. Select the Project Task on which you wish to start the timer
2. Click on the
START (Timer) icon on the top right
3. Click
Pause button in the middle to pause and reactivate the timer
4. Click
Stop button, in the end, to stop for the day
5.Click the
Edit button beside the timer to change the timing and enable
Is Billable button to Yes if you want to bill.

6. Click
Save7. You can view
time log details in the detail view of the project task
Refer :
Timelogs