Familiarize yourself with the MailerSend API by reviewing its documentation. Identify the endpoints that provide the data you need. This typically includes authentication methods, rate limits, and the structure of the API requests and responses.

Install necessary tools and libraries for making API requests and handling data. For a Python-based solution, you would need `requests` for HTTP requests and `pymongo` for MongoDB interactions. Ensure Python and MongoDB are installed on your machine.

Create a function to authenticate with MailerSend using API keys or OAuth, depending on what MailerSend supports. Store your API credentials securely and use them to generate an access token if needed. This step is crucial for accessing protected API endpoints.

Write a script to fetch the required data from MailerSend using the API. Use appropriate endpoints and handle pagination if necessary. Store the fetched data in a structured format like JSON, which is compatible with MongoDB.

Process the data fetched from MailerSend to match the schema of your MongoDB collection. Perform any necessary data transformations, such as renaming fields or changing data types, to ensure compatibility and integrity of the data.

Establish a connection to your MongoDB instance. Use the `pymongo` library (if using Python) to set up a client connection, specify the database, and define the collection where the data will be inserted.

Insert the processed data into your MongoDB collection. Use insert operations such as `insert_one()` or `insert_many()` based on the size of your dataset. Ensure to handle exceptions and confirm that the data has been inserted successfully into the database.

By following these steps, you'll be able to manually move data from MailerSend to a MongoDB destination without relying on third-party connectors or integrations.