To set or remove flags in the Manual Month View or Manual Complete View, click on the date number in the upper left corner of the appropriate date box. 

A new window will pop open in front of the calendar. The jobs and corresponding to that date are listed with call-jobs grouped at the top, work-jobs grouped at the bottom. Flags are shown next to each job name. Click on the Clear Flag to have the Job flagged on the calendar.

After you make your selection, the window closes, the calendar refreshes, and a Red Flag appears next to the Job name. When a Provider is selected to fill the Job, the Red Flag will be replaced with the Provider's initials.

If you need to mark a string of dates as Flagged, use the Manual Schedule Year View.

You can remove Red Flags in the Month View the same way you posted them, or use the "Erase Flags" button in the top right corner of the screen.