First, ensure that your working environment is ready. You need access to the SFTP server, Python installed on your local machine or server, and access to your Google Cloud project with Firestore enabled. Install necessary Python packages like `pysftp` for SFTP operations and `google-cloud-firestore` for interacting with Firestore.

Use the `pysftp` Python library to establish a connection to your SFTP server. You'll need the server's hostname, port, username, and password (or SSH key). Test the connection to ensure it works correctly.
```python
import pysftp

sftp_host = 'sftp.example.com'
sftp_username = 'your_username'
sftp_password = 'your_password'

with pysftp.Connection(host=sftp_host, username=sftp_username, password=sftp_password) as sftp:
print("Connection successfully established.")
```

Navigate to the directory containing your data files on the SFTP server and download them to your local machine. Use `pysftp` to list and retrieve files.
```python
remote_directory = '/path/to/data'
local_directory = '/path/to/save/data'

with sftp.cd(remote_directory):
file_list = sftp.listdir()
for file_name in file_list:
sftp.get(file_name, localpath=f"{local_directory}/{file_name}")
```

Depending on the data format (e.g., CSV, JSON), you'll need to parse and prepare it for insertion into Firestore. Use Python libraries like `pandas` for CSV or the built-in `json` module for JSON files.
```python
import pandas as pd

for file_name in file_list:
file_path = f"{local_directory}/{file_name}"
data_frame = pd.read_csv(file_path)
# Process the data frame as needed
```

Set up authentication for Google Cloud by creating a service account and downloading its JSON key file. Set the environment variable `GOOGLE_APPLICATION_CREDENTIALS` to point to this file, allowing your Python script to authenticate with Firestore.
```bash
export GOOGLE_APPLICATION_CREDENTIALS='/path/to/your-service-account-file.json'
```

Use the `google-cloud-firestore` package to initialize a Firestore client. This step prepares your script to interact with Firestore.
```python
from google.cloud import firestore

db = firestore.Client()
```

Iterate through your processed data and upload it to Firestore. Choose the appropriate Firestore collection and document structure to store your data.
```python
collection_name = 'your_collection'

for index, row in data_frame.iterrows():
doc_ref = db.collection(collection_name).document(str(row['id']))
doc_ref.set(row.to_dict())
```

By following these steps, you can move data from an SFTP server to Google Firestore without relying on third-party connectors or integrations, ensuring a script-driven and controlled data transfer process.