Using the adapter with a camera that has filter threads on its lens is simple: attach the adapter's inner ring to the scope's eyepiece, use the appropriate camera adapter ring (sold separately) to connect the adapter's outer ring to the camera, then slip the camera-outer ring assembly over the eyepiece-inner ring assembly and secure it with the locking screw. The DA1 is compatible with Kowa TSN 601, 602, 661, 662, 663, 664 and 82SV spotting scopes and their respective eyepieces.
(If your camera's lens doesn't have filter threads, consider using the Kowa TSN-DA4 adapter, which doesn't require them.)
The focal length of your digiscoping combination is based on the focal length of your digital camera's lens while in use and the eyepiece's magnification. So if you were using a 100mm digital camera lens and a 20x eyepiece, the combination would produce a focal length equivalent to 2000mm.
All ventures in the world of digiscoping require the use of a sturdy tripod, first and foremost. You must take great care to keep vibrations during exposure to an absolute minimum-vibration suppression pads will help quite a bit here. Employing a cable release, electronic release or remote control for tripping the shutter is absolutely necessary.