He was already picking up the gun when Conyo walked around to gunpoint him, so he was in a position to shoot, as the gun was automatically in passive + it's an easy headshot
It's quite time consuming, you have to edit the frames, frame by frame until the outlines match up if the frames move. What I saw a lot in this video is that he made it flicker a few frames so it looks like it shifts but in reality it doesn't really shift that much. At the gravestone I am pretty sure he just made more of it appear every frame.
Its not hard, but time consuming if you want to nail it.