At low rpm (almost idle) engage pto, then bring engine and pto rpms up to cutting speed. Tachometer should have a mark on it where engine rpms will produce 540 pto rpms.
Does tractor have an electric on/off PTO switch, or a lever that you can ease the pto (clutch) on with?
Does Bushhog have a sheer pin/bolt that will snap if turned on too fast, or a pto slip clutch?