The fs 180° is an ollie during which body and board rotate in frontside direction by 180°. Before you learn the fs 180°, you should have mastered the ollie.
FS 180° - STANCE: REGULAR
The placement of the feet is more or less the same as with the ollie. The shoulders are slightly shifted in the direction opposite to the planned rotation of the body so that the fs 180 motion is easier.
The pop occurs in a similar manner to the regular ollie, but while you pull the front foot towards the nose, you should use the momentum from the shifted shoulders in order to rotate the body in the air. Moving your arms can be useful here.
By the time you reach the highest point, the trick is almost pulled off. By now you should have turned by 90° or more.
If the rotation is good, all you have to do is land the trick and roll off backwards (you're riding either switch or fakie now, depending on the placement of your feet).