How do I block off specific times I'm unavailable during my regular working hours?

This is useful when you have unexpected events, previously scheduled events, etc. that are going to occur during your regular working hours, but you don't want it to affect your regular working times in the future. 

Example: Your regular working hours are from 9am to 3pm. You have a lunch that you scheduled with a friend on Tuesday at 12pm, and you don't want students booking lessons from 11am to 1:30pm (adding extra buffer times).