First of all open an image which shows many different lights in a night background. Now make a new layer now set the color of background to black by first and the foreground color set to white. Click the background for the bottom of the left bar and then pressing 'd''. Next select the 'gradient' tool. Make sure to select the style from the foreground to background. Select the 'mode' to 'normal' and 'opacity' to '100%'. Now drag the top right hand corner to the bottom of the left hand corner. Once you do this it shall appear only a grey screen. Now select 'filter', then 'render' and select 'difference clouds'. This shall show an image of lightening clouds. Now from the top menu click on 'image' then 'adjustments' and click on 'levels'. In the new screen move the middle slider to the left and also the slider on the far right towards the left. Now if you see the image you can see the lightening strike in the clouds. Click 'ok'. Now to invert click 'ctrl i' and then it shall look more like lightening. Now select a big brush with color black and hardness as zero. Remove any lightening that doesn't look like natural lightening. If you keep your background completely black then it shall come out very beautifully. Now from the 'layer' select 'screen' which shall overlay the lightening very nicely on your image. Next hit command or control key and change it into a reasonable size to fit the background nicely. Just check that the point where the lightening ends should not be flat. So use a 'eraser' tool to end it like a natural lightening. That's it.