1. Grip the pistol with the strong hand as shown in the first and second photos. Note that when your trigger finger is not going to be on the trigger to shoot, it should be held up on the frame or slide to prevent an accidental discharge. The strong hand creates a gap that you fill with the support hand (noted in the photo). It is important to get the strong hand as high on the back of the pistol's beavertail as you can, putting you in position to better manage recoil.
2. Rotate the support hand forward slightly and fill the gap, shown in the third photo. The photo does not show much rotation on my support hand, but I hope you get the idea: rotate it a little to make it fit the gap. Both thumbs should be pointing forward, but providing no support. For right-handed Glock shooters, be sure to keep your right thumb below the ridge that sits beneath the slide stop lever so that you don't accidentally activate the slide stop or prevent it from activating when it should. The four fingers of the support hand wrap around the knuckles of the strong hand. We teach our students to keep their support hand index fingers off the front of the trigger guard, because if it touches the trigger guard it will inadvertently steer the gun. The support hand index finger is better used to provide more strength to the grip.
3. The fourth photo shows the grip established with the finger on the trigger, ready to fire.
4. The last photo shows that the hands fit together without gaps. This may vary depending on the amount of flesh on the shooter's hands, but if we see a gap there it is often a sign that the grip is "off" or that it has come undone when shooting, which is common with new shooters. Where I stand when I am teaching, behind and to the side of the shooter, I can see the gap clearly if it is there, and the shooter can easily see it as well. Sometimes we use a Sharpie to make a "witness mark" across the point where the base of the the thumbs are touching, to remind them to keep the grip together. (We are sure to ask for their permission before writing on their hands!)
What you cannot see clearly in the photos is the position of the trigger finger: we teach students to pull straight back through the trigger "break" to the stop, using the pad in the middle of the distal phalanx of the trigger finger. I'm kind of proud of all that medical anatomy talk, I just looked it up! The point where we teach them to pull the trigger is in red in the attached drawing. We also teach them to release the trigger only as far as the reset, so that follow-up shots are quicker and there is less movement of the trigger finger.
I am not, by any means, saying that this is the only way it should be done. I am saying that this is what we teach new pistol shooters, and we get good results with this grip.