Ensure all vegetables and herbs are very fresh for optimal flavor and texture. If using raw shrimp for true acid-cooked ceviche, confirm they are sushi-grade and extremely fresh, marinating for several hours until opaque. If using pre-cooked shrimp (common for this style), a shorter marination of 15-30 minutes is sufficient for flavors to meld. Add the cubed avocado just before serving to prevent browning and maintain its texture. Serve chilled, perhaps with tostadas or tortilla chips.