Begin by creating an account on Braintree and navigate to the API settings. Generate API credentials, including the Merchant ID, Public Key, and Private Key. These credentials are essential to authenticate and access data from Braintree.

On your local development environment, ensure you have Python installed. Then, install the necessary libraries using pip. You will need the `braintree` library to interact with Braintree's API and `pymongo` to interact with MongoDB.
```bash
pip install braintree pymongo
```

Create a Python script to configure the Braintree client. Use the API credentials obtained in step 1 to authenticate your requests. Here’s a basic configuration:
```python
import braintree

braintree.Configuration.configure(
braintree.Environment.Sandbox,
merchant_id="your_merchant_id",
public_key="your_public_key",
private_key="your_private_key"
)
```

Use the Braintree client to fetch the data you need. For instance, if you want to retrieve a list of transactions, you can do so with:
```python
transactions = braintree.Transaction.search(
braintree.TransactionSearch.status == braintree.Transaction.Status.Settled
)
```

Format the data retrieved from Braintree to match the structure suitable for MongoDB. This step is essential to ensure data integrity and consistency. Convert data objects into JSON-compatible Python dictionaries if necessary.

Establish a connection to your MongoDB instance using `pymongo`. Here’s a basic example:
```python
from pymongo import MongoClient

client = MongoClient("mongodb://localhost:27017/")
db = client['your_database_name']
collection = db['your_collection_name']
```

With the MongoDB connection established, insert the transformed data. Use the `insert_one()` or `insert_many()` methods depending on the volume of data you're transferring.
```python
for transaction in transactions:
collection.insert_one(transaction)
```

Following these steps, you can manually transfer data from Braintree to MongoDB without the need for third-party connectors or integrations. Ensure to handle exceptions and errors appropriately for a robust data migration process.