My agency supply me with timesheets. I complete 1 timesheet per assignment, get it signed off by the client at the end of each one and they keep one of the copies for their records. I then just take a photo of the timesheet and what’s app it to the agency, and keep hold of them.
Your question is a good example of how no one tells you anything when you are on agency, starting with the agency 
Hope you have a good first shift.