Choose a difficulty level for common threats: lower (white-symboled cards), higher (yellow-symboled cards), or random (both types mixed together). Prepare an external common threat deck and an internal common threat deck with this difficulty level.
Choose a difficulty level for serious threats and prepare external and internal serious threat decks. The difficulty level for serious threats can be different from that chosen for common threats, but in each case, the internal and external threat deck should be at the same level. …