Moving into the ssemi-diagonal grip takes practice and patients.
Moving Into the Semi-Diagonal Grip
With the dice flat on the layout, place your fingers on the dice the same way you would for the three finger front grip. Place your thumb over the back seam. You don’t have to worry if the front fingers are uneven because once you rotate the dice into the diagonal position, the unevenness of the fingers does not come into play.
Now rotate the dice forward by applying pressure with your thumb at the same time as you press forward with your front three fingers. As you apply the pressure, slide your thumb upward almost halfway to the top.
Use your pointer finger to apply a little side pressure to keep the dice together. You want the least amount of skin possible on the dice.
Launching from the layout will help eliminate unnecessary wrist action and help with your alignment with the back wall. On the release, you want the dice to roll off your front three fingers with no help from the thumb. The thumb is merely a guide till the point of release. Maybe these pictures will help.