I need help. I can't create a soft light leaks like in this video: EpicChillstep Music Visualizer - YouTube
All tutorials on Youtube show how to make a really strong / intense ones.
Could anyone send me a links to some tutorials or describe how to do it?
Light leaks are commonly created from capturing them on camera and superimposing them one a video using the screen blend mode. All you are left to do is find the right light leak video. A youtube search on "film leak", "burn film", "light leak" etc. will get you close enough.