In your Rentals United account, under 'Bookings' - 'New Reservation' you can add new reservations.
On the top right you can manage the status of the booking:
Only the status Confirmed will block the property’s calendar. The statuses request and pending do not block the calendar.
By cancelling the booking, the calendar will be opened up again.
You can add guest details and any comments for your internal information. If you use Stripe in Rentals United, you are also able to charge the guests' credit card by filling out the card information and adding the payment.
Selecting the booked property, the arrival and departure date will block the Rentals United calendar and will synchronise to the connected sales channels in case of a confirmed booking.
Note that the property has to be available in order to add a reservation. If you had manually blocked the dates before, you need to make them available first.