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

Open the Parameters form. On the Payments settings tab, enter the following information:

Set a Post-order screen message for free orders. Example: "Your order requires confirmation. Wait for the manager's call."

You can enter the same message after making a paid order.

2. Set the Reject pending orders automatically after... checkbox.

3. Insert some small period of time, like 1 minute.

4. Save it.

As a result, in a minute after the customer will create an order, it will get the Rejected status. The time on the calendar will be free. Manager will receive an order notification message. So he can open the order and manually confirm it.