You must contact support in order to enable this feature.
Once it is enabled, you can access the Customer Booking Portal through the settings menu. Set the parameters here such as days of the week deliveries can be made, how many days in advance the customer can book and so on.
The next step is to set up notifications that are sent out pre delivery. Here you can include a link to the booking portal where your customers can either accept the given date and time or select an alternative, given your parameters previously set in the settings menu.