Begin by familiarizing yourself with the Zendesk Talk API documentation. This API allows you to programmatically access call data, voicemails, and other relevant information. You'll need to generate an API token in Zendesk for authentication purposes. Go to the Zendesk Admin Center, navigate to API settings, and create a new API token.

Prepare your development environment by installing necessary tools and libraries. Ensure you have Python or Node.js installed, along with libraries for HTTP requests (such as `requests` for Python or `axios` for Node.js). Also, install the MongoDB driver for your chosen language to facilitate data insertion into MongoDB.

Write a script to extract data from Zendesk Talk using the API. You’ll need to make HTTP GET requests to endpoints such as `/api/v2/channels/voice/calls` and `/api/v2/channels/voice/voicemails` to retrieve call and voicemail data. Ensure your requests include the necessary headers for authentication using your API token.

Transform the extracted data into a format suitable for MongoDB. This typically involves converting JSON responses into a structured format that matches your MongoDB schema. Consider how you want to structure collections and documents in MongoDB. You may need to parse and reformat dates, numbers, and nested objects.

Establish a connection to your MongoDB instance. This involves using the MongoDB driver to connect to your database. Specify the connection URI, database name, and any authentication details required. Test the connection to ensure it’s successful before proceeding.

With the data transformed and the connection established, you can now insert data into MongoDB. Use the driver’s insert functions to add documents to the appropriate collections. Handle potential errors and ensure data integrity by checking for duplicates or conflicts during the insertion process.

Automate the data extraction and loading process to keep MongoDB updated with the latest Zendesk Talk data. This can be achieved by scheduling your script to run at regular intervals using a task scheduler like cron (on Unix-based systems) or Task Scheduler (on Windows). Ensure your script includes logging and error handling to monitor and troubleshoot any issues that arise.

By following these steps, you'll have a reliable process to transfer data from Zendesk Talk to MongoDB manually without relying on third-party tools or integrations.