The timesheet can be found in the menu Projects -> Timesheet. This is the place where staff can make time registrations on customer projects.
First step is to select the employee using "Select Resource", and then the week you want to register for, either using the calendar on the right side or "Select Week" using the arrows (look at "Next week" in the above screen). There is a line for each project or task you can register time on and a corresponding hour total to the right side. At the row "Actual hours" you can see the number of hours registered for each day, and a week total on the far right. Click save to save any changes (also before moving forward/back in the calendar).