Play a Campaign level in Halo: Combat Evolved that contains Flood forces. You can use the Assault rifle and Pistol for the glitch, but the Plasma Pistol and Plasma Rifle are more recommended. When you spot a Flood Combat Form, shoot off its tentacled arm. When facing an infected Elite on higher difficulties, you may also want to shoot off its head, as this may be used as a weapon from time to time. If the Combat Form is carrying a weapon, shoot off its right arm as well. When dealing with unarmed Combat Forms, only the left arm must be amputated, and the heads of infected Humans pose no threat.