Put on a pair of safety glasses and leather work gloves.
Spray the spanner nut with a liberal coating of penetrating oil to loosen the bond between the spanner nut and the threaded area holding the spanner nut.
Place the tip of the drift pin on the right side of one of the tabs of the spanner nut.
Hit the backside of the drift pin with the hammer to drive the spanner nut counter-clockwise to loosen the spanner nut.
Move the drift pin to another spanner nut tab and repeat the process. The spanner nut may take a lot of hammering to break tit free and you will need to alternate the tab that you place the drift pin.
Spin the spanner nut counter-clockwise to remove it from the threads of the part holding the nut, after it breaks free.
Clean the threads of the spanner nut and the threaded part with the wire brush to remove corrosion and debris from the area, if you will need to reinstalled the removed spanner nut.