Select the right type of sand wedge. Forget anything less than 60 degrees. You need a great deal of loft to produce height on the shot. You want that 60-degree model to have bounce located near the rear of the club head. That way, the club head will slide through the turf rather than bounce off it, while at the same time preventing the leading edge from digging into the ground, causing a fat shot. Virtually every 56-degree model has the bounce positioned too far forward, near the leading edge of the clubface. That causes the club head to bounce off the turf into the middle of the ball, resulting in a low beeline over the green.
Assess your lie carefully. If your lie is fluffy, the ball suspended above the ground but within the grass, you'll need to produce a more level, shallow angle of approach through impact. Otherwise, the club will slide right under the ball and you'll be lucky to hit it two feet. If you're playing off a tight, hardpan lie, on the other hand, you'll need a bit more of a descending blow--though not steep by any means. Don't force the issue. Position the ball off the heel of your leading foot. Align your feet and shoulders open, but aim the clubface at the target. Make a full backswing. On the downswing, don't hurry to swing the club head into the ball. Let gravity do the work. You want "soft" momentum.
Finally you want to strike the ground an inch or so behind the ball. In that sense, the flop shot is like a shot from sand. It explodes off a cushion of grass and turf. You rely on the trajectory of the ball, not backspin, to stop it quickly on the green.