Pinch both thumbs to remove space from your grip
SagutoGolf's first grip checkpoint is to eliminate the space between your thumb and hand at address. He says that space 'creates variables' and lets the club wobble, so on both the lead hand and the trail hand you pinch the thumb flat against the hand until a crease forms and the gap disappears. That pinched, no-space connection is what makes the grip secure.
Channel: SagutoGolf
Watch official source at 0:44