Calendar Booking: how to set available slots?

Hi I would like to set 1 date for my clients and let them book the available hours. For example the 12th of January. I have 6 slots and I want them to be able to choose their own hour.

Sat Jan 12
10:00 am - 11:00 am
11:00 am - 12:00 pm
12:00 pm - 1:00 pm
1:00 pm - 2:00 pm
2:00 pm - 3:00 pm
3:00 pm - 4:00 pm

I want them to be able to choose their own hour but it seems this booking system automatically books the first available hour and does not give the client the option to choose their own.

If anyone knows how this goes please let me know. I have tried everything already and cant seem to make it work.

Please view my website link: https://www.lotjesbabyphotography.com/bookings//