(i know there are a few ways to do this, but i think this is one of the simplest but yet good looking methods)
Anyway, first we open up a new canvas. Size doesn't matter, because this can be used for anything.
Start off with selecting a brush, i used circle (14) at size 3, black.
Draw the shape that you want you lightning bolt to resemble.

Next, select the paint bucket and fill the area to the right of your bolt with black.
There should be a little white line there, which you can get rid of by brushing over it.

Next, we want to blur that. go to filters>blur>gaussian blur. I used 55 horizontal and vertical, but this is a setting that can definetely be plaayed around with to your liking.

Next, we want to add difference clouds. Go to filters>render>clouds>difference clouds. set the maximum detail (15) and the X and Y should be around 10.

you should come up with something like this

Next, we want to make the bolt white instead of black. For this we go to colors>invert. NOT select>invert.

Now, we want to get rid of those clouds. Go to colors>color levels for this. were only looking at the top bar. find the little black arrow all the way to the left, and move it almost all the way to the right, until you think you like how it looks.

Next, we can go to colors>colorize. Mess around with the hue and saturation until you you think it looks good. I like to keep my saturation at max, to make it stand out more

and, were finished!remember, throughout this entire tutorial settings are meant to be played around with, to get unique outcomes :D
results:

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