1. Identify the original group or rule.
2. Locate إِلَّا or another word of exception.
3. Identify the excluded noun.
4. Ask whether the group is stated.
5. If stated, ask whether the clause is affirmative or non-affirmative (negative or interrogative).
6. Apply the correct case rule; do not assume that every noun after إِلَّا must look the same.
With إِلَّا, the mustathna is necessarily mansub only in a complete affirmative exception.