The flop shot was made famous by the mighty Phil Mickelson.
The pros have this shot in the bag. They understand that in order to pull this shot off, you need soft hands. Soft hands require zero tension. The pros also get that in order to pull this shot off, you need to have the right set up.
A flop shot requires an open stance and clubface. This set up produces an out-to-in path that sends the ball up high with a soft landing.
I like to play the ball in the upper center of my stance.
My shaft is inline with my sternum, if not a hair back, and my hands set low.
I take a wider stance for more stability. My legs are quieter in this shot, so a wider stance keeps them still.