First of all, I recommend you reading the McKinsey case book, which can help you better understand the whole interview mechanics and what happens behind the stage.
For PEI questions you can use a specific structure, which consists of 5 components:
1. Situation (background)
2. Problem (Complication)
3. Your approach (how did you manage this problem)
4. Results (Outcomes of your approach)
5. Your learnings (what you take from this situation)
If you can apply this framework to every of your PEI stories and communicate in a top-down way, you would definitely leave a great impression. For more details, feel free to approach me.