All existing reservations must be added before you can connect the channel manager to OTAs (such as Expedia and Hostelworld). Booking.com allows you to pull all existing reservations with the help of a Switch.cm agent. All other reservations, from OTAs or direct sources such as email or telephone reservations, must be added manually.
You can either use the Import function or use the New Reservation function.
If you want to use the Import function, please check the detailed instruction Here.
If you want to manually add some reservations, there are two ways to add new reservations and one way to create new group reservations. Please check the detailed instructions Here.