If the bookig was made on Hostelworld or Hostelworld powered booking engine on your website, the details are imported into your BPO autimatically.
If the booking is a walk-in, or phone booking, or other non Hostelworld booking
From the calendar (top right on the screen), click and drag to select the nights to be booked
Select the room type and the number of beds and click Book Beds
Next, simply add the booking details and fill all mandatory fields: customer name, email and etc, then click Make Reservation
The booking reference is created, and booking is now saved in the system