Most beginners have difficulty hitting their irons solidly, as a result they tend to lose distance, topping or hitting the ground behind the ball. The issues get worse the longer the iron.
First of all, we need to understand that each iron is designed with different lofts and they are designed to make the ball fly up through the air. The most common fault is we tend to cast the golf club in an effort to make the ball fly up in the air but this will result in a poor impact and reduced distance.
This picture shows casting move at impact
Here are some steps to get in to a good impact position with the help of impact bag.
Get into an impact position at the beginning
From impact position make a half back swing
From your half swing position rotate your lower body to get into the impact position as shown on step 1 above while hitting the impact bag.
This will train your lower body to start the downswing in the correct position for impact with your hips and shoulders open to the target and shaft leaning forward. Most of your weight is on your lead foot, while your are making contact with the impact bag.
Once you get the feeling, try to hit some golf balls with half swing then with a full swing.