There can be multiple combinations to hit the ball lower depending on the task in hand. How low would you like this shot to go? Is it 10 feet or 20 feet lower than your normal ball flight to control your trajectory in the wind or is it extremely low to get underneath overhanging trees.
Here are some ways to lower your ball flight. The more combinations you incorporate into the pre-swing set up or during the swing will reduce the trajectory more and more. This can be implemented with any club in the bag (reference here will be 7 iron).
Ball Position - Moving ball position further back in your stance (closer to trail foot) will decrease the initial launch angle of the ball. *Beware, putting the ball extremely far back in your stance can influence swing path and therefore start line and curvature. A tendency, if it is too extreme, is for you to hit a push or draw'/hook
Club Selection - decrease the loft of your club will decrease the launch. E.g. instead of hitting a 7 iron, hitting a 5/6 iron. The ball will launch lower and produce less spin all things being equal.
Grip position - Gripping down lower on the club (a couple inches) makes you stand closer, have a steeper attack angle and reduce your clubhead speed. This will decrease launch and spin as you won’t swing it as fast
Weight - Position weight on front foot 60/70 percent. This will allow the sternum to move more forward and maintain/increase your hands being ahead of the ball at address. This will influence attack angle causing the ball to launch lower.
Swing Length - decreasing swing length decreases clubhead speed and therefore reduces spin. It will also reduce the amount of wrist action you use in a swing and that also reduces spin. This can be great when hitting shots in the wind
Impact - have the goal of having the shaft lean forward (towards the target) at impact. This will result in your hands being ahead of the ball at impact which decreases loft on the club, compresses the ball more and will launch the ball lower
Finish Position - Tiger Woods would always say that if he finishes high, the ball will have a tendency to go low, and finishing low (abbreviated follow-through) will make the ball go lower. This is because it will influence his impact position and loft when striking the ball
Grip - An extreme way of influencing wrists positions is to have the grip in the palm of your hands instead of the fingers. This will influence swing length, wrist conditions and therefore launch angle and spin of the ball.
To summarise, prioritising a few of these factors will be the easiest to create the biggest difference and be a full proof way of flighting the golf ball: Position the golf ball further back in your stance (1), reduce length of swing (5) and try and get hands ahead of ball at impact (6).