The term you are looking for is modus operandi (not operandus).
Modus operandi literally means "manner of working," so you want the genitive of operandus (operandi) because the genitive in Latin corresponds (roughly) to the english preposition "of."
Modus operandi literally means "manner of working," so you want the genitive of operandus (operandi) because the genitive in Latin corresponds (roughly) to the english preposition "of."