I was looking for making flare like Unreal Infiltrator demo, tried billboard, constrain plane,... finally i used particles to make them which allow more flexibility on size. An culling issue remain but the result doesn't look bad...
First step: create the flare sprite. You can find some source from the web, or make it from scratch with your favorite 2d software. A succession of dot & directionnal blur may be a good start.
Now, create a little material in Additive Blend mode using your texture as opacity mask, and constant color as BaseColor with some multiply values to tweak it after. You can also disable Depth test to avoid flare to be cut by other mesh
Because I can't scale a particle with non uniform scale, I decided to use Mesh particle, that's why I import a plane whitch match with my texture ratio to be use as mesh for particles.
The Emitter still pretty simple:
Mesh Data : put the plane mesh you've created. and set Camera Facing as follow:
In Required, just select your flare material:
In Initial size, set the size of mesh particles:
Other properties have to be set as you wish,you can adjust min and max life and scale to add a flickering effect ... also you can tweak Alpha Over Life (in Color Over Life) to make spawn and death of particles more smooth
Your emitter is now setup. instead of putting it into your scene directly, you can create a BluePrint, expose light color parameter and make the flare color change in the same time.
In the exemple below, you can individualy set color light & flare in the same time, you can also change flare power.