Rather than spend a lot of explaining time, have a look at this image on this article titled the Combat Grip, it may help. You will always notice those combat pistol shooters who progressed from S&W Model revolvers to the Hi Power, by the groove left nicely cut in the top of their thumb.
When it hits the fan they naturally went back to crossing their thumbs behind the hammer, and hey ho the slide did what it does best...............carves skin. So look at the guys hold and work your magic around that. In short get a good firm grip on the pistol grip, and build around that grip with the opposite hand ensuring that hand is not impeding the slide or ejection opening. Hope that helps.
The Combat Grip