On your Guide Schedule, you can block yourself off for the times you are unavailable so you can't get booked. Any day/time without a block will mean you are available for a booking.
You can add one Guide Block at a time, or you can add repeating blocks if you have a set schedule you don't want to get booked in. Guide Blocks can also span multiple days at once, if you are out for a week on vacation, for example.
If a Guide has a block for the morning, they will still be available for a booking in the afternoon, as long as the start time of that booking does not fall within the Guide Block.