There are various types of calibration frames ie darks, flats, bias frames, dark and bias subtracted flats and so on. For the purpose of this guide I am only going to cover dark frames as that is all that I use in calibrating my images. Ideally you should use the maximum calibration when processing your images to get the best out of them, I only use darks because of time issues and other commitments.
A dark frame is an exposure of the same length and same binning as your raw exposures but with the telescope covered and usually several are taken and then combined using median combine or similar before subtracting from each individual raw sub frame. This removes noise caused by dark current in the camera electronics which becomes more evident in longer exposures. Ideally the darks should be taken at the same temperature as the light frames and some cameras have the facility to manually control the temperature of the camera to facilitate this more easily. The darks that I used for this processing were taken on a separate night. There is a useful facility within the calibration of Maxim DL which scales the darks to compensate for any temperature discrepancies. The procedure for taking them is very much the same as for light frames except the software actually tells you to cover the camera before starting. I took 10x20 minute dark frames and used median combine before calibration