A sand iron is a little bit different than the other clubs in the bag.
What makes it different is that a sand iron has an extended trailing edge. The leading edge is the front of bottom of the club. All irons have this edge, but the leading edge of a sand iron extends along the bottom to a trailing edge.
The sand iron is different, so you have to swing it differently. To effectively use a sand iron, you have to use the trailing edge.
What I teach my students is to imagine looking through the face of the club and seeing the trailing edge. Then you want to thump the trailing edge, the bottom of the club, off the ground.
As you're making contact with the ball, the bottom of the club should bounce off the ground. With a sand iron, you don't dig into the ground.
This technique should help you hit more accurate shots around the green with a sand iron.