I want to check every order prior reserving time on the calendar. What is the workflow in this case?

Go to the dashboard and click Payment processing. Enter On-screen message after placing an order. Example: "Your order requires confirmation. Wait for the manager's call."

Then go back to the dashboard and click Statuses. Create a new order status. Enter a name, for example, "Request". Pick status colors. The Blocks time checkbox should be left unset. Save the status.

All new orders should immediately receive the "Request" status. To do this, it should be the first in the list of statuses. Click the Reorder button and drag the status "Request" to the topmost position. Click Save

As a result, when a customer creates his order, the available time on the calendar will not be blocked. After checking the order details, you will be able to go to the order and set the status "Confirmed" to block the time on the calendar.