So what do you need? Well 2 thing: first Paint.NET and next the fire effect plugin. (There are plenty of references on the Paint.NET website how to install plugins, buts it’s basically copy-paste it to the effect directory).
So here it goes:
1) Draw some random stokes in any direction. My choice was to make a Christmas tree on its side.
2) In Paint.NET select Effect -> Blurs -> Gaussian Blur. Play around with the values until you get something that looks like this:
3) In Paint.NET select Effect -> Render -> Fire!… Again, play with the values a bit until you get something that suits your needs.
4) And now the tricky part: duplicate the layer. In the “Layers” window select the layer you want to duplicate (the layer with the fire that is) and press the Duplicate Layer button. Now double click new layer (probably it’s named “Layer 2”) and set the mode to “Additive”. You should get something like this:
5) With the new layer still selected Effect -> Blurs -> Gaussian Blur. Again play around with the values.
That’s it! Crash course “Muzzle Flash” in a flash! Now go make your own!