Normally Employees log in and enter their own work time preferences (prefer and dislike work times), you can also allow them to enter their own "cannot work" times by changing that option on your SETTINGS>Global Employee Permissions page.

These preferences can be used later by AutoFill to create the best work assignments each week. Employees can also log in and request "time off" which you as the Manager can approve or deny. Approving time off removes them from any conflicting shifts automatically.

If you prefer to enter all those Cannot Work times yourself you can do so by clicking the employee name to open the Employee details page and then on the second tab, Schedules - Permissions, you can click to enter first their Repeating Weekly Preferences. Set those days and times that are their regular preferences and then if you have preferences for a specific date that is different you can click the button to Set Date Specific Preferences.

Note: WhenToWork does not create shifts based on Employee preferences, it assigns shifts based on those preferences.