Explain how fault block mountains are formed

Fault-block mountains are formed in areas of rifting. Rifting is where the crust spreads. Where rifting occurs, you will have a fault line, on one side of the fault, the crust will sink and other the other side, it will rise.
The sinking side is called a graben and the rising side is called a horst. Horsts usually form the mountain ranges called fault-block ranges

examples include the Grand Tetons
Mount Holyoke Range of Massachusetts
many of the ranges in the basin and range region in the American Southwest

Fault-block mountains are formed in areas of rifting. Rifting is where the crust spreads. Where rifting occurs, you will have a fault line, on one side of the fault, the crust will sink and other the other side, it will rise. 
The sinking side is called a graben and the rising side is called a horst. Horsts usually form the mountain ranges called fault-block ranges