I hit the flop shot only as a last resort, and amateurs should do the same because it's hard to execute. If you don't really commit to hitting it, you'll either blade the ball across the green or chunk it. But if you absolutely have to loft the ball high and stop it quickly, then you want to feel that the clubhead is passing the shaft at impact.
This release of the club is crucial to generating enough clubhead speed and loft to get the ball up and land it softly. It's important to set the clubface open at address (pointing right of the target), which adds more loft, and then grip the club -- in that order. You can even weaken your grip, your hands rotated toward the target; that will soften the shot even more. But the key is to open the clubface before you take your grip.
Assuming your lie is fairly flat, you can play the ball forward an inch or two for extra height. From there, your thought should be to slide that open face under the ball as the clubhead passes the shaft.
Stewart Cink has won six PGA Tour events, including the 2009 British Open.