If I had seen this first instead taking a few hours trying to draw lightning on my last art post… 
An alternative way you can do the final step is copying the lightning layer you already drew, recolor the lightning layer underneath, set it to Add, then set to Gaussian Blur. If not bright enough, just double this layer again and merge the two.
Airbrush is good too tho who dont wanna be bothered with the mechanics of digital art (I prefer the other due to glow consistency)