There are a few different ways to trill on mandolin. The most common way is to use your index finger and thumb to pluck two strings at the same time, while quickly alternating between them. This produces a fast, vibrating sound that is perfect for adding decoration or emphasis to a melody. Another way to trill on mandolin is to use your pick to rapidly alternate between two adjacent strings. This method is often used for faster passages, as it can be difficult to maintain a consistent rhythm using your fingers alone. Finally, you can also trill using a combination of both picking and fingering. This provides the widest range of options for creating different sounds and rhythms. Experiment with all three methods to find the one that works best for you.