As an administrator go to Settings --> Time Card Approval Settings.
You can then enable time card approvals. If you already have closed pay periods you will have the option to exclude them from the approval process by clicking on "Approve all XX closed time cards now." If you do not do this employees will be asked to submit their time cards from prior pay periods. If you do not have prior closed pay periods you will not see this option.
Once this option is saved all employees with time cards will now be required to submit them for approval. Managers assigned to employees will be able to approve time cards submitted. Administrators are also able to approve time cards.
Here are some additional articles that relate to the time card approvals feature including how to approve or reject a time card:
How do I send out reminders for time card approvals?
Approving or Rejecting Time Cards (Manager or Administrator)
How do I check the status of my time card from the app?
How do I submit my time card for approval?