Start by reviewing the RingCentral API documentation to understand how to authenticate and access the data you need. Familiarize yourself with the endpoints available, the data formats returned (typically JSON), and any rate limits or restrictions.

Create a RingCentral developer account and set up an app to obtain your API keys (Client ID and Client Secret). This will allow you to authenticate your requests to the RingCentral API. Ensure your app has the necessary permissions to access the data you wish to transfer.

Implement OAuth2 authentication to obtain an access token. This involves sending a POST request to the RingCentral OAuth endpoint with your credentials (Client ID and Client Secret) and receiving an access token in response. This token will be used in the header of your API requests to authenticate them.

Use the access token to make authenticated GET requests to the appropriate RingCentral API endpoints. Extract the data you need from the JSON responses. You may need to handle pagination if the data is spread across multiple pages.

Convert the fetched JSON data into a format suitable for MongoDB. This typically involves transforming JSON objects into MongoDB documents. Ensure the data structure aligns with your MongoDB schema to facilitate smooth insertion.

Use a MongoDB driver compatible with your programming language (such as `pymongo` for Python) to establish a connection to your MongoDB database. Set up the connection string with the required credentials and database information.

Use the MongoDB driver's insert functionality to write the prepared data into your MongoDB database. Ensure you handle exceptions and errors gracefully, and verify that the data has been correctly inserted. You might want to implement logging or other verification steps to confirm successful data migration.

By following these steps, you can manually migrate data from RingCentral to MongoDB without relying on third-party connectors or integrations.