First of all, do not ever hit a flop shot unless it is your last and only option to get the ball near the hole. The flop shot is an extremely difficult shot to pull off and control, so I recommend exploring all options before settling on a flop shot from around the green. If there is any way to run the ball up to and on the green I would suggest hitting a chip or pitch shot. The only times when a flop shot is advisable is when you have an obstacle between your ball and the hole, such as a bunker, water hazard, tree, etc. and you have little green to land on and let your ball roll towards the hole.
To execute a flop shot you need to take into consideration many variables. First, you are going to need to examine the lie of your golf ball. This is very important because you are going to play the shot differently if you are in short grass, long grass, buried in rough, etc. If you have a clean lie in short grass you will be able to put more spin on the ball which will help the ball stop once it hits the green. If you have a buried lie in the rough you won’t be able to impart spin on the ball and will have to allow for your ball to roll out once it hits the green. Once you have figured out what kind of lie you have, you will want to walk up to the green to see how much putting surface there is between the edge of the green and the hole, you will also want to look at the contours of the green to see where your ball will roll once it lands on the green. Lastly, take a look at what is in between you and the green. If there is a water hazard or bunker there, you may want to think about erring on the side of hitting the ball too far to make sure you get over the hazard. The last thing you want to do when you short-side yourself near the green is compound a mistake and throw away more shots.
After surveying the situation and determining what severity of flop shot that is needed, it is time to setup to the ball and execute. To set up for a flop shot you are going to want to open your body so that everything aligns to the left of the target. You will play the ball just ahead of the middle of your stance and take a fairly wide stance because you want a solid base for the swing and it helps get you lower to the ground which will help open the clubface. Turn the toe of the club towards your back foot (to the right for a right handed golfer) so that the club face is in an open position, this adds loft to the club so that you can hit a high and soft shot. The next two adjustments are the key to hitting good flop shots. The first is when you turn that clubface open, make sure to take your hands off the grip and re-position them as if you were normally gripping the club. If you fail to do this you will essentially be gripping the club as if it were square because you haven’t moved your hands at all. The second adjustment is to pull your hands back toward your back hip. This again adds loft to the clubface and helps you hit the ball high. Your hands will be behind the ball after this adjustment, which goes against almost everything taught in golf but it is allowed for only flop shots. The key, though, is that your hands will be back in front of the clubhead and ball when you are at impact because you want to strike the ball with a descending blow.
Now that we are setup, it’s time to make a swing. You will need to vary the length of the swing depending on how far and high you want to hit the ball. Always make sure you are accelerating through impact. It also helps to take the club back to the outside of your hands because that will again add loft to the clubface. You are going to need to break your wrists and hinge them quite a bit to get clubhead speed and hold the clubface open through impact to send the ball high in the air. Swing the clubhead along your body line, which is aimed left of the target because the open clubface will send the ball off to the right of where you are swinging (for right handed golfers). At impact you will want to feel like you are sliding under the ball with an open clubface. Be sure to finish your swing and hopefully, if everything goes well you will be watching your ball fly high in the air towards the hole over all the hazards in front of you.