You just tap the rear sight, prefferbly with a brass hammer, in the direction your bullet is hitting. If it hitting to the left tap your sight to the left. It is alway good to put an index mark on the sight and gun so you know where you started so that you have an idea of how far you moved your sights.
Actually you take a brass punch(a long brass rod with smaller end) and move the rear sight in the direction you are wanting the bullet poi to go. If it is hitting right,you move the rear sight left,if it is hitting low,you move the rear sight higher.