Ask yourself: Am I making one choice OR another (mutually exclusive)? Add. Am I making one choice AND another (both required)? Multiply.
OR = Sum Rule = Add. AND = Product Rule = Multiply. That's the entire framework.
Example: Pick a dessert (cake OR ice cream, 3 + 4 = 7 ways). Build a meal (main AND side, 5 × 3 = 15 ways).