Notice - The original image size was 300*200 px. You might have to adjust some of the brush-settings yourself, if you think that they are too big or small.
= Toggle Layer Palette
=
Paint Brush
First create two new layers by pressing the Toggle Layer Palette button and then the New Layer button. Select to work in the top layer. Press the Paint Brush button to use that tool. Set the brush size to 9 and pick the color white from the palette. Start painting the main beam of the lightning. When that is finished set the brush size to 4 and start painting some smaller beams pointing outwards from the main one. After that paint some even smaller ones with the brush size set to 1. Now follow this guiding through the menus: Image > Blur > Gaussian Blur. Set the radius to 0,65 and press OK.
Select to work in layer number two. Choose the color you want on your lightning. In this tutorial I used blue or RGB: 0,0,255. Be sure to select the strongest tone. Set the Paint Brush size to 11 and start painting under the main beam. Then star painting under the other ones using the sizes 7 and 4. After that use the Gaussian Blur with the radius set to 10.
Do the same procedures as in step 2, but use a brighter tone of the color you choused. I used RGB: 159,161,255. One more thing, don't forget to change the Gaussian Blur radius to 3.
Voila, a lightning!
This is the finale result.