If it's a viscous fan, that is driven from the engine mechanically through the hydraulic clutch on the rear, you need a slim spanner to fit behind the fan, then a hammer to knock the spanner, leave the fan belts etc on, helps to hold it still, if slipping on the belts stuff a rag between belts and the pulley to stop the turning, most are left had thread, so take account of that.
tabetha