Concentrating with your Bahtinov mask is easy. Start by snapping the Bahtinov mask into an inexpensive UV or Skylight filter that matches your camera lens. Filter not included, must be purchased separately] Center a pretty bright start into your camera and achieve a coarse focus, then screw the filter that holds the mask on your camera lens. Take a short exposure. The image of the star shows diffraction spikes. The set "center" will likely be moved to one side or the other. This shows the defocus. Adjust the focus of your camera and release the star again to show the diffraction spike pattern. Iterate the process until you get the central diffraction spike against the star and the other spikes. When you achieve a symmetrical pattern, you have achieved the best possible focus. Remove the mask, centre your target object and take your pictures!