Begin with creating or importing an image to the library for which the lightning can interact. Place the image on the stage where desired.
In Properties, change the background to black, creating a nightscape.
Select the image and convert to symbol, choosing movie clip. This is a good time to name the symbol something appropriate to the movie clip.
Double click the newly made symbol, and now begin working in the instance of the symbol. Name the layer. Go to the timeline and place keyframes in frames 2-7. Add a frame at around frame 60.
Select frame 2, then Filters tab, choose add filter, pick adjust color. Move the Contrast and Saturation sliders positively, somewhere between 25 and 45 seems to work well. Repeat this for frames 4 and 6. This is done to produce an effect to “light up” the image when the lightning flashes.
Return to Scene 1, name the existing layer. Create a new layer, name it lightning. On the new layer, use the pencil tool in pencil mode with fill and stroke set to white. Within the Color Tab, place the alpha control at about 50%. Draw on the stage what would appear to be a lightning strike to the image you originally placed.
Select the lightning strike, and convert to symbol, choosing movie clip. Naming this movie clip lightning is a sensible choice.
Double click the lightning strike to enter into the instance for the movie symbol. Place new keyframes on the timeline in frames 2-7. Select and delete the lightning strike from frames 3, 5, and 7. This is done to synchronize the original image “flashes” and the lightning strikes. In frames 4 and 6, delete the lightning and redraw in a slightly different position. This should give the movie an effect whereby the lightning can “dance” in multiple strikes.
Return to Scene 1. Select the lightning, go to Filters tab, and then add any glow or shadow effects you may wish to give the lightning more or less realistic properties.
This simple to create and use tutorial provides plenty of room for expansion and timing cues.