This guide provides a step-by-step process for building user authentication APIs and HTML pages in an Astro application using Lucia Auth and Postgres powered by Neon. The process covers provisioning a serverless Postgres database, creating a new Astro application, setting up the database connection and schema, enabling server-side rendering with Vercel, defining the Astro application routes, building the user authentication routes, deploying to Vercel, and testing the authentication setup locally. By following this guide, users can implement user authentication in their Astro applications using Lucia Auth and Postgres powered by Neon.