It is assumed you already have setup the room type with the number of beds, if not, create this first. See the details of how to setup DORMS, Hostels https://wiki.beds24.com/index.php/Hostel_and_Retreat_Setup
Create a new room with a quantity of 1 in SETTINGS -> PROPERTIES-> ROOMS->SETUPwith Unit Allocation = one per booking.
'''Individual Bed'''