To set up new Web+App Google Analytics for a cross-platform web app, follow these best practices:

1. Create a new property in Google Analytics specifically for your app. Ensure it’s set up for both web and app tracking.
2. Implement the Google Analytics SDK for mobile apps to track user interactions effectively.
3. Utilize enhanced measurement features to automatically track events like scrolls, outbound clicks, and site search.
4. Define custom events for specific user actions that matter to your business goals.
5. Set up user-ID tracking to analyze user behavior across devices.
6. Regularly review and update your tracking setup to adapt to any changes in your app or website.