If you have a Leica spotting scope with the 20 - 60 X zoom eyepiece, you will need to make a ring adaptor with an outer diameter of 40mm, inner diameter of 31mm and about 5mm thick. Basically this is just a thin spacer to keep the lens from hitting the scope lens and for centering the camera lens with the scope lens. I made mine from a lowly plastic milk bottle cap and a piece from Kodak film canister. The shooting is done afocally, i.e. capturing the image with a camera that would otherwise be reaching your eye. For this you must first focus the scope on the spotted bird and place the camera against the said ring and shoot.
Composition is via the LCD screen on the back of the camera.
However, there are some settings best suited for bird photography through the scope. Shutter speed must be sufficiently high to stop motion and camera shake. I normally shoot with Aperture Priority with the aperture opened fully to f4.0 and let the camera set the highest shutter speed possible. The Nikon 990 even have 5 focusing brackets found in the professional Nikons. This is an useful aid for focusing subjects that are not smack right in the center.