Begin by familiarizing yourself with the xkcd API. The xkcd webcomic provides a JSON interface for accessing comic data. You can retrieve the latest comic by making a GET request to `https://xkcd.com/info.0.json` and specific comics using `https://xkcd.com/[comic_id]/info.0.json`.

Set up a programming environment to work with APIs. Ensure you have Python installed, as it will be used to fetch and process data. Additionally, you need the `requests` library for making HTTP requests and `google-cloud-firestore` for interacting with Firestore. Install them using pip:
```bash
pip install requests google-cloud-firestore
```

Set up a Google Cloud project and enable Firestore. Install the Google Cloud SDK and authenticate by running:
```bash
gcloud auth login
```
Create a Firestore database in Native mode and note the project ID, which will be used in your code to connect to Firestore.

Write a Python script to fetch data from the xkcd API. Here's a basic example to fetch the latest comic:
```python
import requests

def fetch_xkcd_data(comic_id=None):
url = f'https://xkcd.com/{comic_id}/info.0.json' if comic_id else 'https://xkcd.com/info.0.json'
response = requests.get(url)
if response.status_code == 200:
return response.json()
else:
raise Exception(f"Failed to fetch data: {response.status_code}")

latest_comic = fetch_xkcd_data()
print(latest_comic)
```

Use the Firestore client library to connect to your database. Initialize the Firestore client in your script:
```python
from google.cloud import firestore

db = firestore.Client(project='your-project-id')
```

After fetching data, process it if necessary and store it in Firestore. Here's how to upload the xkcd comic data:
```python
def upload_to_firestore(data):
doc_ref = db.collection('xkcd_comics').document(str(data['num']))
doc_ref.set(data)

upload_to_firestore(latest_comic)
```

Test the entire process by running your script. You can automate data fetching and uploading by setting up a cron job or a scheduled task. Ensure your script handles errors gracefully, logs activity, and performs retries if necessary.

By following these steps, you can directly transfer data from xkcd to Google Firestore without using third-party connectors or integrations.