This video below shows where the left thumb should be. Both thumbs should be pointing at the target. I like this video but I have to say that as well as showing you how to grip a pistol, he also inadvertently shows you something you should never do. Never allow the muzzle of the pistol to point in an unsafe direction.
He’s pretty sloppy about his muzzle control, I guess because he is facing a video camera and not his students. But even so, he must be willing to destroy his video camera otherwise he wouldn’t allow his muzzle to cover it. Sometimes this is necessary to obtain the desired photo. If you value the camera you would check 14 times to be sure the gun in unloaded. He even allows the muzzle to cover part of his own body at one point. So watch the video for two good lessons. First, how to properly grip a pistol and second, how NOT to allow the muzzle to cover anything you are not willing to destroy.