Add flour in 4 separate portions, a cup at a time, mixing well after each cup.
Knead the dough until it forms a firm ball. It should not be crumbly.
Lightly spray your shortbread pan with a non-stick vegetable oil spray.
Place the ball of dough in the center of the pan. Using the heals of your hands, firmly press into the pan. The dough will be stiff. Prick the entire surface with a fork.
Bake for 30-35 minutes, or until lightly browned.
Cool in the pan for 10 minutes before you loosen the edges with a knife, and flip the pan over onto a wooden or plastic cutting board. If the shortbread doesn't come right out, hold the pan upside down about 2 - 3 inches over the cutting board and drop it face down. The shortbread should drop right out.
Cut into serving pieces with a thin sharp knife while still warm.