To add a flash in After Effects, start by creating an adjustment layer.
Select the adjustment layer and add the Lens Flare effect at Effect>Generate>Lens Flare.
Move your lens flare by dragging the center where you want it.
Press the stopwatch for Flair Brightness in the effects controls panel to start animating the effect.
Press the U key to reveal the keyframe for Flare Brightness in the timeline.
Turn the flare brightness up, then skip a few frames back and make the brightness zero, then skip a few frames forward and make it zero. This should make it flash.
Next, go to the server and look in the Action Essentials folder to find explosions, blood splatters, and smoke. I use the Smoke_Charge videos with alpha. Alpha is transparency.
Drag the effect video you like into the project window and then down to the timeline.
Position and scale it so it’s how you want it and maybe even animate opacity to help make it blend in or out.
When you’re finished in After Effects, add to render queue, remember to change the output to name your file and where it will be created.
Bring it into Final Cut, add sounds and a title, and put it in the Apparate folder on the server when you’re done, post to Youtube, and then to your website.