It was real simple actually. I just took a wire that would plug into the 12V input at the bottom of the camera, cut it off and hooked up each end to the pos and neg terminals on a 12V marine battery. If the pos and neg sides were hooked up backwards, when you turn the cam on it would give you a low battery signal (because you are actually pulling juice from the camera). If you hooked it up correctly, the cam would turn on normally. And yes, you must have batteries in the camera at this time. What I found to work nice is to use low batteries in the cam, so when you turn on the cam you get a low battery signal. Then hook up your external battery, and if done correctly that low battery signal will be gone.