When the ball is buried in the sand your strategy must be different than for a normal lie. You have no chance to slip the club under the ball and so the precision of your shot is lessened.
Go through the following ideas and then try some for yourself.
1. Square up your body to the target line at address and plant your left foot firmly into the sand. Keep at least 75% of your weight on your left foot
2. Close down the clubface (before you take your grip) so that the leading edge aims well left of the hole
3. The swing motion is very aggressive and abrupt. Swing your arms very vertically up your chest in the backswing and fully hinge your wrists. In the downswing mirror the backswing, swing the clubhead down into the sand.
Do not try to follow through to the finish but allow the momentum of the downswing to die into the sand.
The ball should jump out of the sand and run out. At first be satisfied with getting out, but if you are looking to be a little fancy out of buried lies then read on!