First, you need to export the data from MongoDB into a format that can be imported into Teradata. Use the `mongoexport` tool, which is part of the MongoDB tools package, to export the data into a JSON or CSV file. For example:
```
mongoexport --db yourDatabase --collection yourCollection --out data.json
```
This command will create a JSON file named `data.json` containing the data from the specified collection.

If you have exported the data in JSON format and Teradata requires CSV for import, you need to convert JSON to CSV. You can use a script in Python or another language to parse the JSON data and write it into a CSV format.

Ensure that your Teradata Vantage environment is ready for data import. This includes having a suitable database and table structure to receive the incoming data. Define the schema in Teradata that matches the structure of your CSV or JSON data.

Use secure copy protocols like SCP or SFTP to transfer the exported data files (CSV/JSON) from your local system to the Teradata environment. Make sure you have the necessary permissions to upload files to the destination server.

Before loading data into the main table, it's a good practice to load it into a staging table. Use Teradata's `FastLoad` or `TPT (Teradata Parallel Transporter)` utilities to import the data from the CSV file into a staging table. Here is a basic example using `FastLoad`:
```sql
.LOGON your_teradata_server/username,password;
.BEGIN IMPORT MLOAD TABLES your_staging_table;
.LAYOUT your_layout;
.FIELD field1 * VARCHAR(100);
.DML LABEL DML_Label;
INSERT INTO your_staging_table (field1) VALUES (:field1);
.IMPORT INFILE yourfile.csv FORMAT VARTEXT ',';
.END MLOAD;
.LOGOFF;
```

Once the data is loaded into the staging table, perform data validation checks to ensure data integrity. Compare row counts and data types between the source MongoDB data and the data in Teradata. Use SQL queries to verify that the data matches your expectations.

After validating the data in the staging table, you can move it to the final destination table within Teradata. Use `INSERT SELECT` SQL statements to transfer the data from the staging table to the final table, applying any necessary transformations or cleansing operations during the process:
```sql
INSERT INTO your_final_table (column1, column2)
SELECT column1, column2
FROM your_staging_table;
```

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