Begin by creating an API key in Chargebee to access your data. Log into your Chargebee account, navigate to the "API Keys" section under "Security Settings" or "Developers" tab, and generate a new key. This key will be used for authentication in your API requests to Chargebee.

Determine the specific data you need to migrate from Chargebee to PostgreSQL. This could include customer information, subscription details, invoices, etc. Review Chargebee's API documentation to understand the endpoints and data structures.

Write a script in a programming language of your choice (e.g., Python, Node.js) to fetch data from Chargebee using their API. Use HTTP requests to interact with Chargebee's API endpoints, passing your API key for authentication. Handle pagination if necessary, as Chargebee might limit the amount of data returned in a single request.

Prepare the data obtained from Chargebee to fit the structure of your PostgreSQL database. This may involve renaming fields, converting data types, or restructuring nested data. Ensure that the transformed data aligns with your PostgreSQL table schema.

If not already done, set up your PostgreSQL database and create the necessary tables to hold the Chargebee data. Use a database management tool or SQL commands to define the schema that matches your transformed data.

Use a database client library (e.g., psycopg2 for Python) to connect to your PostgreSQL database and insert the transformed data. Write SQL `INSERT` statements in your script to add data to the appropriate tables. Ensure you handle any potential errors, such as connection issues or data conflicts.

After the initial data transfer, verify the integrity and completeness of the data in PostgreSQL. Compare samples of data between Chargebee and PostgreSQL to ensure accuracy. Once validated, consider automating the process using cron jobs (for Unix-based systems) or Task Scheduler (for Windows) to run your script at regular intervals, ensuring data remains up-to-date.