1.pull shirt up with strongside fingers directly over the gun...no thumb just fingers
2. When hand clears gun, jam hand down on the gun in firing grip. This tends to tuck the garment under the gun.
3. draw gun as per normal. As soon as it clears leather start moving it foreward of course. Off hand grabs the gun nice and early or is fending off BG.
This works with my work shirt, button downs, and loose tshirts. YMMV.
