To add availability for a staff member manually on the schedule screen, 

Find the employee and the day you want to make unavailable and select the 'mark as unavailable' button

You can enter a Full day as Unavailable. Full Day's will appear red on the schedule

Or Partially Unavailable. Select the times the staff member is unavailable between and then click save. 

Partial day Unavailability will display in the Peach/Orange colour.

You can copy availability in the same way you can copy a shift. 

Select the copy button within the Unavailability marker and paste the availability where you want.