Update as of May 2020, there is an updated way in which to set staff working hours. It is detailed here https://help.cogsworth.com/en/articles/2383581-how-do-i-set-working-schedules-with-many-breaks-throughout-the-day-and-multiple-shifts-throughout-the-week
Head over to Staff > click on staff member (or add a staff member) > mid way down the page you will notice "Working Hours" > use the radio button to toggle to "custom for this staff member" > toggle days on and off, adjust hours and add breaks by clicking the + sign
NOTE: As of march 2019 you can now set custom service working hours which override your business and staff working hours by heading into your service > working hours. This is an addon.
Below is a video on setting custom working hours for services: